I think one of my external hard drives (Asuna) is failing, it's been playing up for a while. I've *just* got enough room on my other drives to back everything up, so I've decided to do that while it'll still let me access the files, though even that has taken multiple attempts and a lot of patience. The files don't appear to be corrupt, at least. Afterwards, I'll see whether I can salvage the drive by formatting and/or running that disk repair program I used to fix my other drive (Makie/Sayo, still working fine over a year later despite relatively heavy usage).