You may want to take a look at this thread:
https://www.mobileread.com/forums/sho...d.php?t=253478
To make a long story short: if your situation sounds similar, approach Kobo or the vendor about it if your unit is still under warranty. It isn't worth fiddling around with disk images unless you've done something that would void the warranty (e.g. attempted to go inside to fix it or modified the firmware).