Quick update - For some reason I'm in a loop that won't start (or repair, or reinstall) windows 11, but I had an old windows 10 install on the drive. Managed to get into that and run USB Tree viewer and....Nothing.
However, Device Manager was more helpful there, and it turns out that it was an HDMI connection between computer and TV which was the connecting/disconnecting issue! As soon as I can get Windows 11 working again, I think it's sorted. Pity it won't even start in safe mode...