There are many companies that produce great FireWire and even USB 3 drives that go up to 4 TB for single drive units and much more with a drive array.
Check out the Qx2 drive at OWC.
Also, consider using Crashplan Pro E for backup. It may not as pretty as time machine but is far more capable.
You can even combine it with a SuperDuper! clone to an equally large drive.