You can backup your files with Time Machine on any internal or external hard drive, as long as the hard drive is properly formatted. To format a hard drive for Time Machine:
Launch the Disk Utility.
Select the hard drive you want to format for Time Machine from the list of available storage devices in the sidebar.
Click the Erase button and choose Mac OS Extended (Journaled) to format as a Mac volume.
Click Erase again to format the hard drive.