Split a hard drive for use by either Windows or Mac

When you buy an external hard drive for a Mac, it comes preformatted as HFS+ (so that it's unusable from Windows) and when you buy one for a PC it generally comes preformatted as either FAT32 (with a 4GB per file limitation) or NTFS (so that a Mac can only read from it but not write to it). But if you're like me -- running both Windows and Mac operating systems (whether physical or virtual machines no longer matters), you may have wondered if it is possible to split a hard disk drive (HDD) to allow half of it to be used with Windows XP/Vista as an NTFS partition, while the other half to be used with Mac OS X as an HFS+ partition.

I recently used these steps to split up my new Western Digital My Passport Studio 500GB and it works great (I love the Firewire 800 speed)! Here is stock photo of it:

In this article I won't cover the NTFS-3G driver that can add read/write NTFS capabilities to Mac OS X. I also won't review MacDrive by MediaFour which allows Windows to read/write Mac-formatted partitions. The most practical solution that I've found which doesn't require any extra software is to divide up the drive -- one physical drive that presents different partitions depending on which operating system you plug it into. This is successful when you follow the correct sequence of partitioning and formatting the drive -- the order of the steps is important!

1. On the Mac, use 'Disk Utility' (under 'Applications > Utilities') to select the external drive. On the 'Partition' tab, use the 'Volume Scheme' drop-down to split the drive into 2 Partitions. In the first area, select the format as "Free Space" -- this won't actually create a partition, but will set aside an unallocated area at the front of the disk. There is no point giving this area a name since we'll be naming it later in Windows anyway, but do set aside the number of MB (megabytes) that you want to be available to Windows.
   
   
2. In the second area, select the format as "Mac OS Extended (Journaled)", give it a name, and let it have the remaining space (size in MB = Mega Bytes).
   
   
3. Use the 'Options' button to choose the partition scheme as "Master Boot Record". This is the key to having the drive recognized by Windows. Although the Mac OS can't boot from this partition scheme, it can recognize all of the partitions too and our purpose here is to support backups not booting clones.
   
   
4. Click the "Partition" button to appy your changes once you're sure that you've done all of this to the correct drive! Here is a screen shot example:
   
   
   
   
5. Eject the disk from the Mac (drag to Trash and remove connecting cable).
   
   
6. Plug the drive into your Windows machine, and open "Computer Management" (right-click on "My Computer" and choose "Manage" from the contextual menu). Another way of launching this is with 'Start > Run > compmgmt.msc'.
   
   
7. Identify the drive that you've inserted (e.g. Disk1 if Disk0 is your only internal HDD). Right-click on the "Unallocated" space with the black title bar on it and pick "New Partition". Notice the 2nd portion of that drive is a "Healthy (Unknown Parition)" -- that's your Mac partition, so don't touch it.
   
   
   
   
8. Step through the "New Partition Wizard" to format it as a "Primary Partition" with the desired size in MB, assign a drive letter (e.g. "E:"), format as file system "NTFS" and give it a name to help you recognize it. Leave the optional check-boxes de-selected (we don't want a quick format or compression). To begin the formatting, press the [Finish] button on the last screen of the wizard.
   
   
   
   
9. Once the NTFS formatting is complete, the status will say "Healthy" and you can "safely remove hardware" (right-click on the aptly-named icon in your task tray) before you unplug the drive from your Windows machine.
   
   
10. Now you can use the drive with either Operating System! When you plug it into your PC, you'll get the NTFS partition available through the drive letter that you specified, and you won't see the Mac side at all. When you plug it into your Mac, you'll see both partitions, and can view/open the files in the NTFS partition if you want to, but more importantly you'll have a normal Mac partition that you can use with Time Machine or for other backup purposes! Here is a combined set of partial screenshots from the footer of Disk Utility showing how I split up my drive:
    

Did I mention that I love Firewire 800 -- it really is much faster!