Just had another thought.. some people have had issues related to the number of image files that the Kobo Touch creates as it adds books to itself.
For each book you get (I think) 5 distinct image files made; of these (as far as we know) the
are not needed.
Now, when you have more than a certain number of files (some claim 7500) in the images directory, problems occur.
So try this... Add books in chunks of 500. After the Touch has processed those books, go and drop the unneeded images. See https://www.mobileread.com/forums/sho...d.php?t=174388
and take a look at the batch file attached to post 4. You might have to alter it if your Kobo is on a drive other than L.