Time Machine supports all Mac OS Extended (Journaled) formats and Xsan formats. If the disk isn't using the correct format, Time Machine automatically prompts you to erase it when you connect it to your Mac:
If you need to erase the disk manually, use the Disk Utility app to erase using the Mac OS Extended (Journaled) format.
Time Machine can't back up to an iOS device or disk formatted for Windows.
And if your disk uses the Master Boot Record (MBR) partition scheme, some of its partitions might not be available for use with Time Machine.