I've got a 40 gig usb external hard drive that is in NTFS format. I only need to use this with my mac (10.3.9), so I have tried using disk utility to reformat the drive. The drive shows in the disk utility window, but when I select it, and click 'erase' the screen is 'greyed out' and I have no options. I read somewhere that someone had solved this by 'unmounting' the drive, but even when I have tried this the screen is still greyed out. Any ideas?