XBOX hard drives are encrypted to make it more difficult for people to put in a newer, better, and bigger drive. This process is termed locking which requires an encryption key which can decode the information on the drive. Not all harddrives support locking, so check out this database if you are going to upgrade: http://xboxdrives.x-pec.com.
Xbox hard drives, like laptop hard drives, are locked. You must unlock it to use it in another xbox or pc, and it must be locked to be used without a modchip. You can lock a hard drive on any box, but you need the xbox that locked it to unlock a hard drive. unless you have the eeprom