You can force an error 12, which will unlock the drive for about 15 seconds.
Pull the IDE ribbon from the dvd, leave the yellow cable hooked.
Leave the ide ribobn to the hd, and wait until the white MS appears, then preform the hotswap.
It takes time & practice.
Next time backup the eeprom for him and place it on a XboxHDM cd and you wont have this trouble.
... I fscking Love you! that trick saved me from a botched up hotswap I was doing to fix an error 16 problem I had after a UDE2 install. While doing the hotswap I failed to plug in the orginal IDE cable to the HDD, I ended up powering down the xbox then remembered that I should have reconnected the orginal IDE cable to the HDD... :idiot: So that left me with a error 21 and a locked HDD (confirmed with Xboxhdm)
So to make this clear again... I'll send you some Russian Brides or something XD