SD card fault concern/or my error

[Tahli]

Hoping you might shed some light on a little mystery I have, this may be too confusing to follow (I am trying to work out what may have happened or if I have a faulty card). I won't be concerned if you say "odd, who knows":

Nikon camera - Duel cards 128GB - second card slot set to back up

Thought I'd reformatted both prior to session - but afterwards I realised that card in slot 2 had not been reformatted and had 2 other older sessions still on it (about 64GB full)

A few shots from end of shoot card said full so I took both out and replaced with one card and took final shots but thought it very strange they would be full

Copied over shots to HD (about 64GB) and have been editing

Then last night I went to format both cards for a new shoot and put in camera and *they did not now register as full* I thought that was odd so I looked an realised one card had the 2 older shoots on and not the latest, other card had latest shoot and was about half full. So it sort of appears that the second card didn't operate as a back up or hit capacity as the back up, as it was already half full, and then the current shoot was deleted. Is the later possible? Or could the card have thrown an error? I checked the settings and second card was definitely set to back up. Tested with a new photo that it was writing to both cards. I wondered if I'd 'cut' the shoot off the card when copying to HD but I don't think I used that SD card - I usually drag them to a folder.

Probably just an anomaly and I'll never really know but concerns me a little if the card if faulty. Maybe it's a user error, probably won't know what happened for sure and I'll keep a closer eye on this to see if similar happens again. I don't like when things happen that I can't explain, hope you understand that (haha). I have all the photos so that's the main thing, thanks for considering this. Tahli