This is such a good question. It's a question that I hope everybody asks of themselves.
Data is never safe unless it exists in at least two different places. So yes, if your photos are on an external hard drive, you need to have a constantly-updated mirror copy of that drive's data on either another drive, or on a cloud service.
Having two drives means that if one drive fails, it's very quick and easy to get up and running again from the other drive. But it leaves you vulnerable to disasters such as fire or theft.
On the other hand, keeping your files on a cloud server means that they're safe from fire or theft at your house; but of course the file transfer is significantly slower.