If you want to use the external hard disk for both OS X and Windows, the Volume filesystem should be FAT(32) and the partition scheme MBR.
Remember that FAT(32) has limitations. Mainly it cannot handle files larger than 4GB. If the Windows machine is running Vista or later you can use ExFAT instead. You cannot use NTFS because OS X can only read NTFS volumes.
In order to do that you need to reinitialize it, so save whatever is on the disk now and use Disk Utility or the Terminal.
From Terminal issue
diskutil list
to find out which disk is the external one. Let's say the command above returns /dev/disk1. Now issue
diskutil partitiondisk /dev/disk1 MBR ExFAT VolName 100%
that means, partition the whole disk (100%) using MBR partition scheme and ExFAT filesystem. Give it the name VolName
You can change ExFAT with fat32 if you want.
I never formatted disks so large with an alien, i.e. not Apple, filesystem. I know older version of OS X couldn't do that. But you can give it a try 😉
If you prefer Disk Utility, here's the visual explanation
Choose the Format scheme from the drop down menu, MS-DOS (FAT) or ExFAT
