You can't format it like that. You need to delete it's partition first. Create a new partition. Then format that new partition. So you need some 3rd party partition software if you don't have the means already. Another easy way to do it is to pop in the HDD, grab your WinXP or Vista disk, and boot from it, and when asked, act like you're gonna install it new and use it's built in partition/formatter to repartition and format that drive, then cancel the install, take out the cd, and boot normally.