I just upgraded the hard drive in my Macbook. I was planning on selling the old hard drive today but hadn't formatted it yet, so I took it to work to use an XP Machine to format it. Disc Management and Partition Wizard won't let me do anything. Every option is greyed out, and I noticed it shows type as GPT. How can I format this from a windows machine as I don't have time to go home and format before meeting the buyer.