[DNSB]

It's not the 10,000 author names, it's 10,000 book filenames in the root directory. Since most people include the title and author in that filename, they tend to be longer than would fit in an 8.3 filename. Add in series which seems to be the most popular addition for non-Kobo owners so the filename would become (using a book on one friend's device) "Amaranthe: Machine [03] - The Stars Like Gods - G. S. Jennsen.epub" which would chew up 6 directory entries. One of the other books on their device is an anthology for which they included all the authors in the filename so it clocked in at 214 characters which chewed up 18 directory entries—and that was after calibre truncated the filename. With 8,200 books in the root directory, they ran out of available directory entries. Moving their library into a directory off the root directory, adding the author sort as a subdirectory in the library directory and trimming both the author sort and author to the first name in the list corrected their issue. Basically, change the driver's save template to "Books/{author_sort:sublist(0,1,&)}/{title} - {authors:sublist(0,1,&)}". We then restored the metadata and bookmarks saved in calibre before the move and they are a happy camper now.

Trying to convince them that they did not need to put the series in the filename since that was how they did it on their old ereader was whole other can of worms. Just send the books, wait for the books to import, reconnect to your computer and let calibre send the metadata for the books.