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 p