truinstinct, I just added a 120gig to my friend's box using the exploits. It's very easy to do if you have a modded xbox handy.
If you've tried adding a larger harddrive to the UNModded box, you'll notice it won't boot. This is because the larger hd is UNLOCKED. So in order for it to boot, all you have to do is find a way to lock it. It's a bit tricky to lock a hard drive on an unmodded box because you dont have access to the software on the unlocked harddrive, nor can you boot a CDRW on the unmodded xbox.
Using a modded box, I simply installed the larger hard drive onto mine. After doing so, I loaded up ConfigMagic. Now, If I lock the hard drive immediately, the larger harddrive WILL NOT be usable in the UNModded xbox because each Xbox has a UNIQUE HDD Key.
So in order for the hard drive to work in the unmodded xbox, you're going to have to update the modded xbox's EEPROM with the UNModded EEPROM. Doing this will give the modded xbox the same HDD key used in the unmodded xbox. After updating the EEPROM on the modded xbox, you can then lock the larger hard drive.
Once it's locked, you can now boot the larger harddrive on the unmodded xbox. And you're set. You can then do the exploits and such. I hope this did more help than harm, but feel free to PM. Goodluck.