[geekmaster]

Your USB drive partition might be damaged, but it *should* be reformatted and repopulated with factory default files if it cannot be mounted, during startup (even in diags mode) if you read the startup scripts.

Does SSH still work? You could try running the factory_restore script others have used, as mentioned in the "Select Boot" thread.

If your /mnt/us is not mounted, you can mount it manually with:
mount -o rw,offset=8192 /dev/mmcblk0p4 /mnt/us

Or you can do this on a host linux PC:
sudo ifconfig usb0 192.168.15.201
nc -l 5555 < mmcblk0p1.img

and then on the kindle:
nc 192.168.15.201 5555 | dd of=/dev/mmcblk0p1 bs=4K
It can take up to 30 minutes to write that large mmcblk0p1 image to mmc.

You have many options you can try, if you are patient and persistent, and want to invest the time to learn how to fix it. Of course, these new kindles are so inexpensive that it would be a better use of your time to just buy a new one, but then were is the fun in that? It just feels better to fix it yourself, and you learn stuff while doing this. Even the failed attempts teach you something.