OId PSD files opening with no layers, just <background>

I've noticed recently that when I open PSD files from several years ago, 2012, 2011, 2010, they are opening flattened with a single 'background' layer. Photoshop doesn't bring up any errors, just opens the files with a single layer.

Initially I thought it was just the odd file, but I've gone back and looked at several from lots of different shoots and they all have the same problem.

Now I absolutely know that there would have been layers in the files, there's no way all those files would have been saved as flat PSDs. 

I've tried a couple of things:

• opened them in CS6 and CC2020, both flat
• renamed them to PSB, still flat
• Moved the files to a local internal dtive and opened from there, still flat

These files are on an external USB 4Gb hard drive, the were moved from a desktop onto the USB hard drive last year when I moved country. I can't imagine why but could they have flattened in transfer??

Files created in Photoshop in Windows and opening them in Windows.

I have no idea what to do, I've got useless PSD files with no clue as to what the original edits were.

Anyone else had a similar issue?