If the halo 2 method dosent help I'd use xboxhdm 1.9, "that only uses cd/dvd though" and it Sounds like your backup of the C: is a backup of the shadow c: though.
Id try to get the c: files from a working box. "soft moded or not" but make sure yor not backing up the shadow c:
Unfortunatly i cant give you the c or e files that you need because they are MS's property. But they can be found on some rescue disks..
if you can simply add the numbered save folders to the udata folder on E you should be able to soft mod, if you have been accidentaly copying the shadow c over, the soft mod should still see your updated dash folder and continue.
error 13 and 21 arent so bad they are dashboard errors. It shouldnt be too hard to fix..
P.S
Its not a bad idea to have a xboxhdm disk with working files on it ready it dosent have to have the eeprom data on it. and then you can make a 4gb dvd with the c: and e: drive and have some apps on there as well. And you can use that to prepair all your hard disks, then have xboxhdm cd's with only the eeprom data fo each drive on them for locking the drives afterwards.