To do this on a mac:
1. Open up "Disk Utility"
2. Select the drive that needs to be formatted on the left.
3. Click on the "Erase" tab to the right of the list of drives.
4. Select the Volume Format from the pop-up list. FAT 32 if you want to use it on windows AND mac.
5. Name the hard drive. Type a name for the disk in the field where it asks for the name. This name will always appear when the disk is mounted on a Macintosh computer.
6. Click the erase button. A window will open asking to verify the erase procedure. Click the erase button in this window to start the process of erasing the disk. Once the process is complete, the drive will appear with the name it was given on the left side of the window with the list of drives and on the desktop of the Macintosh computer.
7. Enjoy