Sxpartax check drunkmonk's post above. Maybe you have a bad memory card, which would have caused your problem in the beginning.
Sxpartax do not extract gz file gdiskdump works on zips.
I am not a Linux guru and managed to copy the image to my stock Kobo card (so it was OK)
The gdiskdump program is not a command line program. In the menu just pick input
--- file not disk (your source gz file) click forward and select output
disk (make sure it is your SD card. Go, no formating required. You can do the exact opposite to backup your Kobo.
If this doesn't work I would go Drunk monks route --get a new card, maybe new card reader and adapter. Really desperate try the Windows program
My wife's Kobo Vox has been running perfectly since I reinstalled the image and made two different backups.
I can't get it away from her to do any experimenting