If your Kobo ebook reader suddenly stops responding due to too many ebooks you may want to try the SQLite Database Browser, which is a great piece of open-source software, to cleanup the Kobo’s database by removing all references to any ebooks. Since you only remove the references the ebooks will still be available, but the Kobo is forced to reindex the ebooks. For a clean shot you may want to move the ebooks to your harddisk before cleaning up the database. Maybe it’s also possible to force reindexing by just deleting the database, but I guess this will be the same as resetting your Kobo to factory defaults, which is not desired, and I haven’t tried this, yet.
2014-08-22 @ 10:09: How to fix your Kobo ebook reader if it’s not responding anymore