You can't use a harddrive in the xbox at all unless it is locked and keyed to whatever the eeprom is looking for.
Bingo. That is exactly why I tried to flash my eeprom. I softmodded my xbox and cloned to a larger drive. Everything went smoothly. So smoothly a friend asked me to mod his xbox. So I grabbed a cheap hard drive online for $15, cloned my mod over to the hard drive and it booted up nicely...from my xbox. Tossed it in his xbox and got an error (07 I think).
Fine, I thought, I'll just soft mod his stock drive and clone over to the larger drive again. Modded his xbox and tossed in the drive. I/O error while attempting to clone. Something about bad sectors (didn't happen on my xbox). So I figured I'd just flash his xbox with my eeprom since the larger drive worked fine on mine. Flashed my eeprom from stock just to see if I could do it with ConfigMagic before messing with his. Worked fine, until I rebooted into ComaConsole--no video, no audio, no eject. Fan ramps up during boot-up.
I thought maybe I did something wrong, so I was extra careful as I flashed his eeprom with my eeprom.bin. Comatose.
In hindsight, his drive was likely locked onto my eeprom/motherboard, which is what I thought the eeprom-flash was intended to work around.
So now I'm stuck with two comatose xboxes that should work perfectly fine, and two expanded and softmodded hard drives ready to play. That's why I'm trying to find a way to reflash my eeprom (and his) with the stock eeprom. Can't find the steps or software to do it though.