This CNET TV video gives you step-by-step instructions on how to set up your external hard drive so it can be used on a Windows machine and on your Mac. Keep in mind that the process described in the video is using Mac OS X 10.5.x Leopard, though the process is essentially the same in Mac OS X 10.6 Snow Leopard.
The best format for maximum compatibility is FAT 32, easily read by Macs and the default for Windows-based PCs. To format your external hard drive to FAT 32 from your Mac:
- Plug in your external drive. Keep in mind that by formatting your drive you will lose all your information.
- Open Disk Utility (Applications > Utilities), select your external drive, and click the "Erase" tab.
- Under "Volume Format" choose MS-DOS (FAT), which is FAT 32, and click Erase.
- Press Erase again to confirm and let it do its thing.
Again, please be sure that you do not need any of the data on your external drive (or that it is securely backed up), since the reformatting process will completely erase the drive.