Originally Posted by jackie_w
This problem was discussed by the Calibre developers when the T1 was new. This post
and this post
may shed some light.
I have some doubts as I NEVER put a book on an SD-card (I only temporarily tried to put some fonts there when I tried to references fonts not embedded in ebooks nor inside the PRS-T1) and I still had this problem (so no possible book number conflict in the SQLite databases)