[jlynton]

Thanks David, I will do as you suggest.

[edit]I shortened the file path from "{author_sort[0]}/{author_sort}/{title}/{authors} - {title}" to "{author_sort[0]}/{author_sort}/{title}", then removed all the books from the device. After re-loading them the problem still exists. The .kobo/images directory maxes out at 8371 files (about the same as before, so shortening the route does not seem to have helped).[/edit]