Using Time Machine for backup requires a volume that is at least 2 or 3 times the capacity of the computer's HDD. But as I said earlier if you don't need to use it on Windows and wish to use if for backup on the Mac, then prep it exclusively for the Mac. But you can re-prep the drive using GUID partition scheme for two partitions and format one of them for MS-DOS or ExFAT to use on Windows. Because the drive was formatted on a Windows system you cannot partition it via the Mac.
Unless you need to retain the Windows files you have put on the drive and copied to the computer, then I suggest you simply re-prep the drive for use on the Mac.
