[Milhouse]

Switching to the conservative governor would make sense when performing an update, though that wouldn't prevent the ext4 partition from being trashed which is the other piece of the puzzle (though not so much if you're booting from USB/NFS).

I don't know what or why the mmc0 errors are occurring, but if you boot with an overclock, experience the mmc0 errors, then switch to the conservative governor does that magically mean the SD card is behaving itself (until you switch back to the ondemand governor)?

Ultimately, preventing the mmc0 errors in the first place is the only solution.