Back up any data you can from the HDD.
On your Mac, use Disk Utility and create 2 ea. 1 TB partitions. Choose MBR ( Master Boot Record ) as the Partition Table. Format one of the Partitions as MS-DOS name it whatever you like. Format the other Partition as Mac OS Extended (Journaled). Boot up Windows and plug in your drive. Go to "My Computer" or "This PC" and format the MS-DOS partition as NTFS. Simple. Now you will have the Mac partition for OS X and NTFS partition for Windows.
Windows will "Read-Only" the Mac partition.
Mac will "Read-Only" the NTFS partition.
This is handy for file swapping.
Cheers!!