So - looks like with the help of Florian, I've fixed my disappearing artwork issue. 
Now - is there a way to restore artwork from a back-up drive? That is - can MP3Tag search on artist & title in a specified folder, and copy artwork from the backed-up file to the current file?
Otherwise, It'll be a case overwriting all of the files with missing artwork from my archive drive... Rather not do this, because it would mean losing some of my more recently added text/tags.
