[GeoffR]

From what I gather the FAT32 filesystem used on the Kobo's user-accessible partition has a limit of 65534 "entries" per directory, and a file with a filename of length N occupies 1+N/13 entries (rounded up). So for example a file with a 50 character filename would occupy 5 entries, and so no more than 13,106 such files could be created in a single directory.

The default file naming scheme in Calibre results in quite long pathnames. For example the book "The Door Through Space" by Marion Zimmer Bradley would be stored as:
"/Bradley, Marion Zimmer/Door Through Space, The - Marion Zimmer Bradley.epub".

The Kobo device stores the cover images for this book in three files named similar to:
"file____mnt_onboard_Bradley,_Marion Zimmer_Door_Through_Space,_The_-_Marion_Zimmer_Bradley_epub - N3_LIBRARY_FULL.parsed"

That is 3 files of about 120 characters each, with the files for all books stored in a single directory /.kobo/images/. If that was a typical book then the directory will only hold images for about 2000 such books.

While space for 2000 books might be plenty for those with a 2GB micro SD card, it is likely to become a problem for those of us who have replaced the stock card with a larger one in the hope of storing more books.

I have an 8GB card in my Glo and hope to have more than 2000 books some day, so I guess I will need to choose a more efficient file naming scheme than the one Calibre uses by default.