@sugarfree aha! That's a great shout!
Sorry for the delay in replying - been a busy week and just had time to try.
In this time, the Pi has crashed once or twice, so the original desire to save progress is now a lost cause. However, I'm hoping I can copy the contents of the USB off onto another, so I can keep srm and roms at least.
I have a feeling that this may be a combo of:
A failing USB drive.
A permissions problem.
With the second one, I did all setup via ssh, including downloading roms. It worked without issues for ages, but I'm wondering if somehow there's ended up being a conflict where the ssh user can't see the contents of the drive, but the user for retropie can.
It may be in reality the same user...! 🙈 I use Linux a bit for web work, but I'm not great at all the stuff for using it as an actual computer.
The other thing that makes me think it could be permissions/access is I have been trying to get a Dualshock 4 controller to connect via Bluetooth. If I ssh into the Pi and run the retropie_setup script as per the guidance, it can connect to the controller, but not do anything in Emulation Station. Running it with ds4drv doesn't work either.
Then I tried to do the same from the Emulation Station front end (Retropie > Retropie Setup) and it worked flawlessly. I can control everything with the Dualshock 4 without any further setup.
So I've tried to access the USB stick via Emulation Station frontend too. Retropie > File Manager. They're all there!
Then on to point 1 above - after around 5 minutes in the file manager I started getting error messages. Have taken a couple of photos of them, as I wasn't able to copy any output. Lots of blocks failed. Lots of references to error - 71.
photos of error messages
I'm using an official Raspberry Pi 5 power supply and only have the USB stick and Bluetooth running, so I don't think there's any power issues going on.
Thank you again for your help and advice here!