Terra external drive crashed

[melkarlberg]

I had someone else set up my Terra RAID external who I thought knew what they were doing.  I thought it was mirrored, so I would be okay in a crash.  This morning, the top light turned red and now I can't find any of my files.  It doesn't appear that it was mirrored.  What kind of troubleshooting can I do to possibly retrieve my 3TB of files?  And are there things I shouldn't be doing (like turning it off and on repeatedly to see if it decided to work LOL)?  I do have Crash Plan and have 95% of my stuff backed up there, but I would so much rather recover it locally rather than try to transfer that amount over my painfully slow internet.   I'm attaching the depressing screen shots that let me know it wasn't working.

After I get past this crash, I would love a link to a tutorial (if one exists) to help me configure this drive to mirror itself.  I spent a ton of money on it for that reason, but I don't think it was ever set up correctly.  I would rather learn to do it myself so I am not counting on other people to protect my files.  

Thank you so much in advance.  

[Brian]

Oh boy. 

If the external drive was setup as a RAID0, which takes at least two hard drives & makes them act as one big HD, and one of those drives goes bad...

...you are screwed. The whole RAID setup is gone. At this point you are looking at extreme data recovery. I recommend this guys: 

https://www.gillware.com/

I would budget around $1000 - $1500 to start. Thank Goodness that you have Crashplan. A 5-7% loss is a lot better than a total data loss.

I've never heard of a Terra RAID external, so I would be reading the manual just like you. I can tell you that you'll know you are in a RAID1 configuration because you will lose 1/2 of your total capacity. The reason is due to RAID1 and how manufacturers market things  two 2TB drives as a RAID0, that's around 4TB total capacity  if setup as a RAID1, that's a little less of 2TB of capacity, after formatting. I'd say about 1.8TB or so.

So while a manufacturer will market that this is a 12TB external RAID drive, that is in a RAID0 configuration. When it's configured in a RAID1, drive mirroring, that's one 6TB HD being instantly duplicated to another 6TB Drive.  

 

[melkarlberg]

1 hour ago, Brian said:

Oh boy. 

If the external drive was setup as a RAID0, which takes at least two hard drives & makes them act as one big HD, and one of those drives goes bad...

...you are screwed. The whole RAID setup is gone. At this point you are looking at extreme data recovery. I recommend this guys: 

https://www.gillware.com/

I would budget around $1000 - $1500 to start. Thank Goodness that you have Crashplan. A 5-7% loss is a lot better than a total data loss.

I've never heard of a Terra RAID external, so I would be reading the manual just like you. I can tell you that you'll know you are in a RAID1 configuration because you will lose 1/2 of your total capacity. The reason is due to RAID1 and how manufacturers market things  two 2TB drives as a RAID0, that's around 4TB total capacity  if setup as a RAID1, that's a little less of 2TB of capacity, after formatting. I'd say about 1.8TB or so.

So while a manufacturer will market that this is a 12TB external RAID drive, that is in a RAID0 configuration. When it's configured in a RAID1, drive mirroring, that's one 6TB HD being instantly duplicated to another 6TB Drive.  

 

Thank you so much for replying.  I actually bought the housing and the three drives separately and had someone set it up for me.  I thought I had it mirrored.  Is there a way to check?  It's my understanding that, if it is, I need to buy a new drive to replace the bad one and it will rebuild itself?  Does this tell you anything about whether or not it was mirrored?

[melkarlberg]

Each hard drive is 4TB.  I don't know if that helps when looking at this picture.  

[Brian]

So it looks like you were setup in a RAID5?

You should be able to get to your files. A RAID5 can have one HD go bad and still have files accessible. Yes, you will need to replace the failed Hard Drive and it will rebuild the new drive for you. Depending on the device, this might be done automatically or you might have to start the process in this software control application.

What happens when you go into Disk Utility? Does the Drive show up there? Have you powered off the device and pulled the failed HD? Looks like it's the third HD that has failed. Drive #1 is "M0," Drive #2 is "M1" and Drive #3 would be "M2." Can you take a photo of the EHD unit? I want to see what it looks like. Error lights and all.

What ever you do, DO NOT CLICK "DELETE ALL RAID."

RAID 0 = Disk Spanning. Multiple Drives are treated as one big HD.

Benefit: Speed. RAID 0 is fast, but only with traditional hard drives that have spinning platters. SSD drives in a RAID 0 configuration have no performance gain.
Downside: When one HD fails in a RAID 0, ALL files are lost. The whole friggin' thing is gone.

RAID 1 = Drive Mirroring. One HD is copied over to a 2nd HD in real-time.

Benefit: When one HD fails, you have a duplicate. So when you replace the failed drive, the new drive will be rebuilt from the duplicated drive.

Downside: If Data is corrupted on one HD, that corruption is copied over automatically to the second hard drive.

RAID 5 = Block Level Striping with distributed parity.

"Huh?"

A RAID 5 is RAID 0 and RAID 1 combined, conversationally speaking. Greater Storage Capacity vs RAID 1 and you can have one failed HD out of three (or more) disks and still function, so it's not like RAID 0 where everything goes ka-plooey when a drive fails. RAID 5 is what you want if you have that option. The downside is that it costs more because you need at least three identical drives to create a RAID5.

------------------
The next step is replacing that Hard Drive that has failed. Hopefully it's still available and yes, you want the same drive / model number.

[melkarlberg]

Thank you so much!

[melkarlberg]

This is what shows on Disk Utility

 

[Brian]

If you were setup as a RAID1, you would have a little less than 4096GB total capacity, so it was configured as a RAID5. (Which is what I would have done.) Because it's grayed out, the macOS won't see the drive. You are still going to have to replace that failed hard disk drive. That's the 1st step and there isn't more I can do for you until that happens.

[melkarlberg]

12 hours ago, Brian said:

If you were setup as a RAID1, you would have a little less than 4096GB total capacity, so it was configured as a RAID5. (Which is what I would have done.) Because it's grayed out, the macOS won't see the drive. You are still going to have to replace that failed hard disk drive. That's the 1st step and there isn't more I can do for you until that happens.

That's what I was thinking.  Is the best case scenario that, once I replace that drive, it will begin to rebuild itself?

[Brian]

11 hours ago, melkarlberg said:

it will begin to rebuild itself?

Depends on the software. Typically, yes. That said, you might have to initiate the process and It's not as bad as it sounds. Kinda like right-clicking on something and selecting, "Rebuild RAID." I'd start reading your instruction manual to get an overview of the process while you are waiting for the drive to arrive. As I mentioned above, I have NO EXPERIENCE with this particular or brand of external RAID HD. I would be reading the manual myself. I have rebuilt RAIDs on fancy and expensive servers though, and it's not THAT big of a deal.

The trick is pulling the correct failed HD and getting the exact same model/size of HD. Or one that is larger. With a RAID, you can get a larger HD, not smaller. Now before you start thinking you'll get more space, you won't. LOL! The RAID will just add the larger HD and rebuild it with the same size partitions. So even though you might replace it with a 6TB HD, it will only format it to a 4TB HD, just like the other two drives.

[melkarlberg]

Thank you so much.  I'll start reading up and order the new drive.  Fingers crossed.

[Brian]

Relax. This type of thing happens ALL the time. There is a logical process that must be followed. If you follow that process, step by step, things will turn out ok. It's when you are freaking out do things go wrong. From what I can tell, this is a Noontec TerraMaster Device? What is the exact model number?

[melkarlberg]

It's a Noontec-TerraMaster D5-300 USB3.0 (5Gbps) Type C 5-Bay External Hard Drive Enclosure Support RAID 5 Hard Disk RAID Storage (Diskless) with 3 of WD Red 4TB NAS Hard Disk Drive.  I've read on here this week that the WD Reds are often an issue, but that's what we bought initially just over a year ago.  Do I have to stick with that brand since that's what's in there already?  

We have a ticket in with Terra customer service to see if we can get some guidance before ordering anything or moving forward.  In searching for online support, I've read some pretty dismal things about their customer service, but maybe my issue is standard enough to not need too much support from them.  

[Brian]

Take a look at the document on this Noontech page: The steps to automatically recover data in RAID 5.

WD RED Drives are meant to be in NAS devices and run 24/7. I personally like the Western Digital Caviar Black Line of hard drives as they are faster, but are a tad bit louder than their RED counterparts. I also think they run a little hotter due to them being faster, but it's not an earth-shattering thing. (7200RPM vs 5400RPM.) Yes, WD RED Drives seem to have high failure rates, but I'm used to things like this. Hard Drives, Tape Drives and Power Supplies are the top three things to fail in servers.

If it were me, I'd probably turn on and turn off my NAS and would use Caviar Black Drives. I'm sure your next question will be, "Can I use the WD Black Drive with my existing Red Drives?"

Short Answer: No.

Long Answer: You need to have three identical hard drives, IMHO, with a RAID5. Adding a faster hard drive with different firmware and possibly different capacity will cause you headaches. Even though you would purchase a 4TB or whatever, the actual capacity could be different. For example, HD 0 and HD1 are 4096.97GB. The new HD, which will be HD2 is 4096.96GB. They are both considered and marketed to be "4TB," but one is slightly smaller; one ends in .7 and the other ends in .6. That .1 difference will cause the RAID to reject that drive and state that it's "Too Small." Yes, I have had this happen to me, multiple times these last 24+ years being a tech. LOL! 

Bottom Line: Hard Drives fail all the time. In fact, I'm replacing one today on a server. So you have two options, replace that failed WD RED drive, or purchase three new Caviar Black HDs and restore from your cloud backup. If it were me, I'd get another RED drive and rebuild it. Time is money.

[melkarlberg]

So, after a week of talking back and forth with the raid manufacturer, it looks like my Mac has turned off permission to the RAID drives, so I need to try to open it on a PC so I can copy/paste to a new drive.  They recommended the following:
please install the MacDrive software on a Windows computer. Once it's installed, the software will remind you to restart your PC, once it's been restarted, you may plug the RAID, and the RAID should be detected in the software and you then can back up your data. Once the data is backed up, you then can delete all the RAID in your Mac and rebuild a RAID 5 by 3 good hard drives. 

Here is my question.  I am going to purchase a new external hard drive to transfer the information.  Since I will have MacDrive installed on the Windows machine, I should be okay transferring the files to this new external and then plugging it into my Mac?  I just need to make sure it is formatted for a Mac and then that I use MacDrive for the reading and transfer?  Does this advice make sense?  I got the feeling the tech was just reading from a manual but not really familiar with the system.  

Could you also recommend a good external drive to use with my iMac?  I am thinking two 8 TB drives (one primary and one that I manually back up every week, so I don't deal with this RAID system any longer).  Or maybe I should get a new 8 TB drive and use the RAID as my backup?  I just want to do this right this time.  I have wasted so much money and time on this RAID, but I am fine scrapping it if that will have me time and money in the long run.

[Brian]

A RAID system is the way to go, if you are backing up critical data. That being said, what you end up fighting with is the stupid software that these manufactures give you that control the RAID and it's not fool-proof. I'm not just talking about the software interface that you get things setup with; I am also talking about the software built into the external enclosure itself. It's a proprietary type of software and each manufacturer is different, especially when it comes to the "consumer level" stuff. So what you are dealing with is crappy programming and "We don't know what went wrong...too-bad-so-sad." Unfortunately, getting a decent commercial-grade RAID unit is not cheap. Personally, I like the RAID Controllers that come with HP Servers. They are Compaq controllers and are the best in the industry, as far as I'm concerned, but they aren't a consumer-type of device.

 

6 hours ago, melkarlberg said:

Could you also recommend a good external drive to use with my iMac?

I like G-Drives. They are expensive though, but it's what I own and they come Mac-Formatted. Here is a 8TB Model with both Thunderbolt and USB 3.0 Ports.

By default, a Mac OS will READ a Windows Drive, but not write to it. So if you are recovering images and files on a Windows machine, it's better to have that EHD formatted as a Windows Drive to restore your files to, THEN hook it up to your iMac. You don't have to get fancy, a simple 2-3TB HD should work. Keep in mind that you want to keep individual file sizes less than 4GB each. After you get things restored to your Mac, then you can format the EHD for use with the Mac. Make sense? 

This HD caught my eye: WD My Book Pro 12TB. I like that it comes with two Western Digital Caviar Black 7200 RPM Hard Drives. (That's the internal HD line that I purchase with my own money.) I would set it up as a RAID1 which is two 6TB Hard Drives that are mirrored.

 

7 hours ago, melkarlberg said:

Or maybe I should get a new 8 TB drive and use the RAID as my backup?

You have that backwards, the WD RAID is hooked up to your iMac.

Use an 8TB EHD to copy the data off the WD 6TB weekly and store that 8TB off site. Heck having two 8TB EHDs and a Week A / Week B Rotation is even better. Now I realize this isn't cheap. I get it...but keep in mind, compared to legal fees, hard drives are cheap. If you are doing anything professionally photography-wise, and I don't care how iron-clad your contract disclaimer is, legal fees / court fees add up quickly if something gets lost and you get sued. Attorney fees of $250+ /hr, paperwork charges, phone-call charges adds up.   Just think of it as and insurance policy with no deductible.

[melkarlberg]

Before I take this drive to someone for recovery, I have a question.  Right now, the tech support is telling me that my Mac cannot read the drive because it won't mount without the degraded disk removed - they have said it won't matter if I put in the new drive because the Mac has restricted the drive.  I've had a few people tell me that I can just pop in that new hard drive and it will rebuild itself.  I'm wondering if I should try that before attempting other forms of recovery.  Other than the $150 for the new drive, is there any disadvantage to trying this?  Could it hurt my chances of recovering it through a professional if I attempt to put a new disk in that bay where the degraded disk was? I want to trust the tech support at the RAID drive company, but it kind of felt like he was just giving me answers from a manual.  

 

I just found out that my off site file recovery company definitely doesn't offer physical drive recovery any longer (which they did when I signed up with them), so I'm more motivated to retrieve it through the RAID if there's any chance, but I don't want to take an action that will affect the drives' possibility of being recovered.

[Brian]

3 hours ago, melkarlberg said:

 I've had a few people tell me that I can just pop in that new hard drive and it will rebuild itself.  

That's the whole point of a RAID, especially a RAID5.

I think I told you above to install a new drive. The other two should rebuild the replacement. The only part that I'm not sure of is that you manually have to trigger it or if the RAID Software will do it automatically.

Macs are funny when it comes to RAID Drives. The OS is Unix-based, which dates from the 1970's AND you are running the standard OS, not the Mac OS Server edition. It just has a fancy GUI that makes things easier. If this was a Windows box, I'm 99% positive you would have gotten a warning message about the failed drive and still would be able to access your files.

I'm gonna give it straight: I feel that you are just driving yourself nuts. Replace the damn drive and be happy. (Same model number, same or slightly larger capacity.) You really do not want me to go on and on about blocks and parity and striping and all the other stuff on WHY a RAID will rebuild the replacement drive. Your eyes will glaze over and I'm stopping myself before I start geeking out. 

[melkarlberg]

10 minutes ago, Brian said:

That's the whole point of a RAID, especially a RAID5.

I think I told you above to install a new drive. The other two should rebuild the replacement. The only part that I'm not sure of is that you manually have to trigger it or if the RAID Software will do it automatically.

Macs are funny when it comes to RAID Drives. The OS is Unix-based, which dates from the 1970's AND you are running the standard OS, not the Mac OS Server edition. It just has a fancy GUI that makes things easier. If this was a Windows box, I'm 99% positive you would have gotten a warning message about the failed drive and still would be able to access your files.

I'm gonna give it straight: I feel that you are just driving yourself nuts. Replace the damn drive and be happy. (Same model number, same or slightly larger capacity.) You really do not want me to go on and on about blocks and parity and striping and all the other stuff on WHY a RAID will rebuild the replacement drive. Your eyes will glaze over and I'm stopping myself before I start geeking out. 

This is what I thought, but I just wanted to double check since the tech had me doing all of this extra stuff :).  Thank you so much for bearing with my hemming and hawing -- I just didn't want to do anything wrong as I try to recover this.  Thank you again :-D

 

[melkarlberg]

1 hour ago, Brian said:

That's the whole point of a RAID, especially a RAID5.

I think I told you above to install a new drive. The other two should rebuild the replacement. The only part that I'm not sure of is that you manually have to trigger it or if the RAID Software will do it automatically.

Macs are funny when it comes to RAID Drives. The OS is Unix-based, which dates from the 1970's AND you are running the standard OS, not the Mac OS Server edition. It just has a fancy GUI that makes things easier. If this was a Windows box, I'm 99% positive you would have gotten a warning message about the failed drive and still would be able to access your files.

I'm gonna give it straight: I feel that you are just driving yourself nuts. Replace the damn drive and be happy. (Same model number, same or slightly larger capacity.) You really do not want me to go on and on about blocks and parity and striping and all the other stuff on WHY a RAID will rebuild the replacement drive. Your eyes will glaze over and I'm stopping myself before I start geeking out. 

Sorry - one more question.  Just got off of my last web meeting with the Terra tech.  He says that 

the RAID as a complete single drive is broken, and it somehow caused the protection from Mac, and then the access authority is restricted by Mac (so the auto recovery will not work). 

My suggestion now is to turn to the professional data recovery company to recover your data. 

 

-- My question is -- I have the RED WD Drive still in the packaging.  Even though he says this, I was still planning to try to insert it and see what happens based on your feedback.  If this doesn't work, I would like to rebuild the RAID and just recover from my remote backup service.  Is there a tutorial that you can recommend for when I start doing that?  Finally, can I get an idea of what a professional should charge me for recovering this data.  I know that varies, but I want to make sure I'm not getting completely taken.  I also wonder if it's even worth it since I have the remote back up.  I would rather it be quick, but I also realize that recovering files sometimes results in crazy file names and such and may result in more time in the long run. 

[Brian]

OK, I've done some digging around. It seems Apple has reduced the functionality with the MacOS starting in El Capitan. I'm still on Mavericks, for many reasons. Disk Utility doesn't manage RAIDs any longer with the current OS and a 3rd party software program is needed for the RAID. So that explains things, this is a "Software RAID" and not a "Hardware RAID." The difference is that with a hardware RAID, the device's firmware has built-in software that manages the RAID. The benefits are if a drive fails, all you have to do is pop the failed HD out and install the new one and it takes care of things automatically. This is what I deal with on a daily basis with my customers and their professional-grade servers. The biggest downside to this is, you guessed it...cost. Hardware RAID devices with auto-recovery are not cheap. With a Software RAID, you are dependent on the Operating System and Software working correctly; so it's not bullet-proof. Things can go wrong and you could get locked out of your files and not be able to do a automatic recovery, which is exactly what you are going through. Ugh.
 

15 hours ago, melkarlberg said:

Finally, can I get an idea of what a professional should charge me for recovering this data.

It's not cheap. At all. I'd budget between $1000 and $1500, conversationally speaking, for this service. I recommend a company called Gillware.
 

 

15 hours ago, melkarlberg said:

I also wonder if it's even worth it since I have the remote back up

That's your call. If it were me, I would install the new HD, and create a new RAID5. Then download from your cloud backup.

You could also give things a whirl and see if you can make an attempt recover with the new drive. Tech Support says no...and I believe them....but who knows? You might get lucky. You are already going to more than likely blow the existing RAID away and setup a new one, so while it's already broke, no-harm-no-foul.

 

15 hours ago, melkarlberg said:

I would rather it be quick, but I also realize that recovering files sometimes results in crazy file names and such and may result in more time in the long run. 

Most definitely. When a HD is recovered, crazy filenames are part of the fun. Though it really depends on who or what is doing the recovering. A professional company like Gillware may make things easier, though at a high cost. Even then there aren't any guarantees, which leads us to "How much is it worth it to you? How much do you really want to spend?"

 

15 hours ago, melkarlberg said:

Is there a tutorial that you can recommend for when I start doing that?

There should be some sort of documentation, I think I linked you to the .pdf file from the company above. Honestly, it's fairly logical.

For example, in the screen-shot above is the RAID software. If you wanted to blow away the RAID, you select the Radio Button that has DELETE ALL RAID next to it and click Apply. After that's done, I'm sure there is something that will pop up asking what RAID you'd like to create. Replace the failed HD and look for something that says, "Create RAID" and select RAID5 and click Apply. This is a pure guess on my part, though it's pretty much what I deal with when I create / delete RAIDs on my Servers that I maintain. It's not rocket-science, all you have to do is pay attention and have some confidence in yourself. It's already broke, you can't make it more broke unless you do something stupid, like push the enclosure off the desk while it's running. Then it will REALLY be broke. LOL!!

Think logically. Input --> Output.

Delete RAID --> Replace Failed HD --> Create RAID --> Follow the Wizard Prompts. It should take you more than 15 minutes total, and that's figuring time to mount the new HD in the enclosure thing/tray.

Please note!!! Your enclosure may not support hot-swapping. To be on the safe side, after you delete the existing RAID, power down the enclosure, THEN replace the failed HD with it off. Otherwise you could blow things up, like the enclosure's back-plane, the power supply or the existing hds. That would not be good.
 

[melkarlberg]

UPDATE 

First of all, thank you so much for all of your feedback on my situation.  Even though I didn't recover the RAID, I felt more calm and confident taking the steps to attempt it.  I got every image back that I lost (thanks to CrashPlan and me not erasing 4 CF cards from the summer yet).  

MY CURRENT SITUATION

I have 2 EHD, organized into a)everything before 2016 (3TB of information - we will call this EHD A) and b)2016-present (a little under 2 TB of information - we will call this EHD B).    I also have the RAID with 4 4TB hard drives.  I haven't done anything with the RAID since I tried to recover and it didn't work.  I bought the new drive to insert into the RAID to try to recover everything, so I've invested way too much in that system to just let it sit there.

I've read a little about RAID 10 systems, which seems like it would give me 8 TB of space that would be mirrored with the remaining two disks.  I probably have about 2TB of useless files to delete on my other two EHD, and this might give me the push to organize those, so 8TB is a realistic amount of space for a couple of years.  My hesitance with this system is that I don't trust the RAID to recover anything if I lose it again.  My next option would be just to use those 4 EHD as 4 separate hard drives in once place, so I have 16 TB of storage space.  That seems like a better option.  I likely could have accomplished this cheaper, in hindsight, but this is what I have now, so I would like to use it.  

If I do set the RAID up as just 4 separate drives, what is that called?  Is it possible?  I would like to set it up so that my 2 existing EHD auto backup to the RAID disks every week - EHD A to one disk and EHD to another (which leaves me 2 disks to grow into).  I know this means I will eventually have 4 EHD backing up to 1 mass storage system, but I'm fine with that.  I know you mentioned using the RAID as the main and back up to the two externals before, so maybe the reverse would make more sense.  I'm just not sold on the stability of my RAID, but I suppose all hard drives fail eventually.  

 

[Brian]

On 7/15/2017 at 8:47 AM, melkarlberg said:

If I do set the RAID up as just 4 separate drives, what is that called?  Is it possible?

It's called JBOD. Which stands for Just a Bunch Of Disks. Seriously.

It works similar as a camera would with two slots, when HD #1 fills up, it starts using HD #2, then #3 and #4. If one of those HDs fail, say like number 2, you can't get to HDs 3 & 4. I do not recommend using JBOD mode. The only people that I know that do that are technical geeks that just do it...because they can. Unless that HD unit has a mode that treats four hard drives separately, and I really-really think that it doesn't, then you possibly could. Personally, I have never seen and enclosure that was meant for a RAID treat 4 hard drives as individual EHDS. I mean, what's the point? You buy that enclosure for it to be a RAID unit, and the only other mode would be JBOD, which again, I do not recommend for your "typical" end-user. 

 

On 7/15/2017 at 8:47 AM, melkarlberg said:

My hesitance with this system is that I don't trust the RAID to recover anything if I lose it again.

That's the main issue. You are looking for a bullet-proof system and unfortunately, it does not exist. ANYTHING man-made can fail. Failed Hard Drives are the number one thing I replace when it comes to servers. Followed by faulty Tape Drive units, and then Power Supplies. I can't tell you how many hard drives I've replaced in the last 25 years being a tech. It's in the thousands, perhaps 10,000 or more. That's why you have a backup of your RAID. Typically, it's either to a Tape Drive, or to a backup appliance / cloud service, which is what my fancy $30,000+ servers my customers do. We have backups of the RAID and then backups of the backups. One is kept off-site at all times.

The only thing that RAID is supposed to do, is give you more fault tolerance when something fails. So if a drive dies, you can keep going. Unfortunately, it seems that your Software RAID isn't as robust as one would like. If it worked as it should, that HD would have failed, you should have still been able to access your data, then replace the HD, and have that HD rebuilt / added to the RAID and gone about your day with little to no down-time. I'm thinking you just purchased something that just doesn't live up to expectations; so rather than blaming RAID for your issues, you should be blaming the manufacturer for selling crappy stuff.

Again, what I would do is create a new RAID5. Use All four drives in that unit, you'll need 3 at a minimum and if you can set the 4th as what's called a hot spare...DO IT!!. What a hot spare does is what the name implies. When the RAID controller software detects an issue with one of the drives, it automatically builds that 4th drive into the RAID setup and then disables the faulty drive. Then you replace the faulty drive and that becomes the new Hot Spare. After getting your RAID 5 setup, get two larger HDs, like two 8TB USB 3.0 units or even 16TB units and backup nightly / weekly backups. Be sure to keep one EHD off-site. You could rotate the HDs nightly, which is the safest route, or do a weekly rotation. Your call.