HI mmax,
I'm not sure if you can use my image on Kobo Mini, but if that's what Mr_Ploppy did probably you can...
I had similar problems with booting from my SD-cards. Try formatting it with SD-Formatter first (it fixed my SD card):
https://www.sdcard.org/downloads/formatter_4/
Enable "Format Size Adjustment" option!
Then write an image to it.
If this won't work - there is another workaround you can override kernel only on working card and copy needed files manually.