Well probably there are better ways to get 4.2.1 but u could follow this guide for a factory reset to stock Rom 4.2.1 http://forums.androidcentral.com/nexus-7-rooting-roms-hacks/191477-guide-nexus-7-factory-image-restore.html
But with that u will loose the clockwork recovery and the root (and...
I am not sure if i can help you so much but i had a similar Problem today.
I switched to clockwork recovery today and i got the same mount-errors. The solution for me was quite easy: I booted it (normal, no bootloader or recovery) and filled all the necessary google-account Details etc. (Like u...
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