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The key to the HDD of your xbox. I recently soft modded my friends xbox. (he was kinda a noob) he pretty much tried to "uninstall" the softmod by deleting all the files in the c: drive (including the eeprom).
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without his specific eeprom his xbox would not accept the hard drive and it crashed. I tried using xboxhdm with my eeprom and it wouldn't work because I didn't have the right key.
Now, which one of you is the noob?
EVERYONE knows you should backup your eeprom key to your PC for safe keeping. Since you're the one who softmodded his xbox... did you make the eeprom backup?
Also, it's common knowledge that YOUR eeprom key won't do a damn thing to help him lock HIS hdd to HIS motherboard. Knowing this, you tried anyway?
Just stick a chip in his xbox and use slayer's or AID 3.0 to rebuild his hard drive, then lock it with ConfigMagic.
When you're done... backup his eeprom & FTP it to a PC (and burn to disc) for safe keeping.