I saw the files, and they don’t look like .aseprite files 
It looks like the content of the files are raw bytes from the disk (not written by Aseprite, e.g. there are filenames inside one of those files).
This might happen because: 1) the disk has invalid sectors (so they are not saved correctly anymore), 2) Windows didn’t flush the content to the disk on time. (Maybe related to this thread, on Aseprite v1.2.8 I’ve added a explicit call to flush/sync the content of files on disk so nothing is lost, but doesn’t look like this case.)
My recommendation is to do a full check of the external disk for bad sectors.