[rms2001]

This was taken from WesternDigital's website and shows why most computers and the XBox will not use all the space in a HDD bigger than 137GB.

Whether or not a given system can support 48-bit addressing (drives over 137GB) is dependent upon several factors. The operating system must support the higher capacities. Windows 98 and higher will support the drives with the proper BIOS, EIDE controller, and driver support. However, since motherboards and BIOSs vary widely, the answer is not as simple as making sure your operating system will support the drive.
Systems which do not support the full capacity of these drives, will generally report the capacity only up to the 137 GB limit. Any capacity above this limit will neither be recognized nor displayed. If your system is capable of supporting the full capacity of your drive, it should be properly displayed in either the CMOS set-up or during Power On Self Test (POST) prior to booting.

The motherboard BIOS, the EIDE controller, and the EIDE controller drivers you are using within your operating system must support the drives as well. It is best to contact your motherboard manufacturer to determine if your motherboard, BIOS, and EIDE controller drivers support drives over 137GB.