Select the drive and select the Partition tab.Open Disk Utility (in Applications/Utilities).I'd recommend that you create the HFS+ partition on your Mac first, then, on your Windows computer, format the other partition to NTFS: You can use one NTFS partition to store backups of multiple Windows computers.) (Why only one NTFS partition although you back up 3 Windows computers? Because Windows stores backups in folders named after the computer name so there are no conflicts whatsoever (see this post). In this case, two partitions will suffice, one HFS+ and one NTFS. Using partitioning you can trick your computer into thinking it is connected to more than one drive, although there's only one.
Luckily, you don't need two separate drives. In the Format menu for the partition that will be used for backups. Make sure 'Mac OS Extended (Journaled)' is selected If you want to partition the disk, click the Partition tab and Manually preparing a new disk for Time Machine Your preferred setup is not possible: OS X can't share an NTFS disk with Windows for backup because Time Machine needs HFS+ (from ):
