1. Connect your hard drive to your mac computer and check whether the drive is powered on.
2. Double Click on Macintosh HDD.
2. Go to Applications > Utilities > Disk Utility.
3. Double click on Disk Utility.
4. Highlight the hard drive you want to format and go to erase button.
5. From the volume format select “MS-DOS (FAT)” and enter a name.
6. Click “Erase” and conform.