Well, in fact, when it comes to the user partition, it's a bit more difficult. That's because the user partition carries a partition header within it, like if the partition was another full disk. At least that is the case for my K3. They probably did that because Windows and other OS can cope better with "Hard Disks" (devices with a partition table) than with "Floppy Disks" (devices without a partition table, containing one filesystem that spans the disk). Plus, in the worst case the user will reformat it, maybe forcing it to get a partition table. This is because the Kindle presents this (internal) partition as a full disk externally via USB.
However, when the full MMC space is exportet as in this recovery mode, the user partition now suddenly carries the file system starting at a later offset.
All this isn't relevant for the OP, of course, since the error indicates clearly that the root file system is the culprit. Running an ext3 file system check should bring more information (e2fsck is the tool needed for that, it does ext3 despite its name).