Thanks to both MrTomahawk and tptech for pointing me in the right direction. Turns out that there are a couple of tricks to this. For more information see the following:
Basically, you first create a share on your WHS for your Time Machine backups to go. I called mine "macbackup" and gave my user full permissions to the folder as well as set it to replicate so that I have a redundant backup of it.
Next, open up a terminal window on the Mac and switch on the magic preference
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Now, mount the share on your Windows Home Server using Finder.
Back in the terminal window, you now have to create a sparse bundle file. Take a look at the links above for more information, but basically you type a command like the following
hdiutil create -nospotlight -library SPUD -size [size_in_gigs]g -fs HFS+J -type SPARSEBUNDLE -volname "Backup of [hostname]" -verbose [hostname]_[mac_addr].sparsebundle
Replacing [size_in_gigs] for the size you want to constrain time machine to use, [hostname] for the machine name of your mac and [mac_addr] for the mac address of the interface you want to use for the bacup (usually eth0 as doing the backup over wifi will take a while)
I used something like the following command:
hdiutil create -nospotlight -library SPUD -size 250g -fs HFS+J -type SPARSEBUNDLE -volname "Backup of martinmbp" -verbose martinmbp_002332c5437e.sparsebundle
Then copy this over to the mounted share using something like
cp -r [hostname]_[mac_addr].sparsebundle /Volumes/mackbackup/.
Now, go to System Preferences, Time Machine and select Change Disk.. Pick your WHS share that you have copied the .sparsebundle to and then leave for a long time if this is your first backup.
Seems to be working for me. I'll report back and edit this answer if I find any issues with it. Mileage may vary as Apple obviously left this functionality out for a reason, I'm assuming it is a technical one rather than a ploy to sell more Time Capsules so we'll see what happens.
Been using for few days so far and seems to be working ok. A couple of caveats:
- Reports of issues if you put the machine to sleep while performing a back-up. I did this as a test and had trouble getting the back-up volume to un-mount. In the end I had to reboot the machine to release the lock on whatever file was causing the issue.
- Problems if trying to restore from Time Machine backup using System CD. Because you have to set a preference to get the machine to pick up unsupported devices like Windows Home Server shares, it is unclear if you can restore a whole machine from the System CD that doesn't have the preference set. Personally I have a few whole image backups that I have kept around that I could restore from an external disk so this hopefully won't be an issue for me - the Time Machine stuff is really just an incremental backup. However I still need to test the restore process
It is unclear to me at this point if the solution provided by the HP WHS servers also has these issues, or even if they are just general Time Machine quirks.