Apple’s time machine is a brilliant app. The only downside is that you can’t (apparently) backup to a network share… thats what I thought, now I’ve found a way.

This worked for me, but ITS NOT A DOCUMENTED WAY (ie: use at your own risk).
1. Mount the network share (I have a share at work on Windows, and at home on Linux – both work).
2. Open iterm and run the following (all on one line!);
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
3. Start time machine preferences, click “Change Disk” and you should see your network share, set as a target as you normally would.
I found that sometimes time machine can “forget” the share, then wont let you backup to it, to get around this reset the parameter using the following (all on one line!);
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 0
Follow step 2 again, and all should work – I guess this is the slightly buggy side of things, and why its an unsupported method. Hopefully an update will address this soon, as its really handy if you already have a NAS/storage and don’t want to spend cash on a Time Capsule.
Enjoy