there are various utils etc for unlocking the xbox hdd, most are for the original seagate/wd 8/10gb drives that the xbox shipped with. I'm trying to hunt down my utils, been a couple of years since i last had to. If its an 300gb drive are you sure its locked? i know for softmodded xboxes they had to be locked, but i know my 200gb in my tsop flashed xbox doesnt need the drive locking...
the following link might (or not) help
http://www.llamma.com/xbox/Unlocking%20Seagate%20Xbox%20Hard%20Drive.htm
or try to find a copy of
http://www.hddunlock.com/
will post more if i can find it
intel i7 3770k @4.4, ASRock Z77 Extreme 4, 16GB Corsiar Vengance 1600 ram, ATI 5850 BE, Antec 1 Case, Antec 1 550W PSU, emu0404, 120gb agility3 ssd OS, 2x60gbvertex2raid0 Games, 3 TB total sata2 Data. win7 x64.