Some hard disks ship with a Master Boot Record (MBR) partition type. For these disks, you will need to change the partition type by erasing the disk, so that it is supported by Time Machine:
Important: Erasing a disk deletes all files on it. Make sure that you have or make a separate backup copy of important files, in another location, first.
- Open Disk Utility.
- Select the backup disk's icon on the left side of the window, which usually appears with a numerical capacity and is offset slightly to the left of any volume icons. If you see a tab named "Partition" appear, proceed to step 3.
- Click the Partition tab. (If you only see tabs named First Aid, Erase, RAID, Restore, then you have selected a volume on the disk instead of the disk itself--repeat step 2.)
- From the Volume Scheme pop-up menu, choose the desired number of partitions (or 1).
- Click the "Options..." button.
- Select a new partition scheme:
- Use Apple Partition Map partition scheme if the disk will be used with Time Machine and a PowerPC-based Mac.
- Use GUID partition scheme if the disk will be used with Time Machine and a Intel-based Mac.
- Click OK.
- Click Apply. This will erase the disk.
Once the external hard disk is repartitioned, select it again in Time Machine preferences and use it for your backups.