Hello,
What you can do is, connect the drive to a Windows computer and then perform a format from the Windows computer. Once that is done, then you can connect the drive back to the Mac, and then you can setup the drive to work on both Mac and PC. This will erase anything that is on the drive, but here are the steps for you to format the drive from a Window machine.
If you right click your Windows my computer icon, or Windows computer icon, click on manage. On the left double click Disk Management. Your drive should show. With your mouse right click the drive,and click format. If you have windows vista or windows 7 you will choose delete volume and then you should get the option to create a new volume.
Let us know if you have any other questions.
