I recently purchased a new 1TB external hard drive for a project. Now that the project is completed I wanted to re-purpose the hard drive for use as a backup drive with my Mac as a target for time machine.

I initially went to the ‘Disk Utility’ application in my utility folder to format the drive. However, each time I’d format it I received an error saying the format system failed when trying to format any of the Mac OS formats. It would format FAT-32 fine but not Mac specific formats.

It turns out that you have to select the parent drive in ‘Disk Utility’ then click on the Partition tab. There select 1 partition. Then click on the options button and select the first item GUID for Intel macs or second item Apple Partition for PPC Macs. Then click ‘OK’. Now click ‘Apply’ button (for Leopard) or ‘Partition’ button (for Tiger) .

You now have a Mac OS compatible external hard drive that can be set to boot your Mac if needed.

The issue is that most external hard drives come formatted as FAT-32 or NTFS with a windows partition. By changing the partition to GUID  or Apple Partition we can format the drive Mac Extended Journaled or any other Mac OS format.