The below link is for the ancient Kobo Wifi, but later models of Kobo also seem to have these data blocks in the raw partition:
http://blog.ringerc.id.au/2011/01/ta...i-without.html
I had similar poor display issues when using a SD image from one Kobo mini on another Kobo mini. Transplanting the data blocks mentioned in the above link from the corrupted SD card to the image from the working SD card not only fixed the display, but also set the mac address.
Something to consider.