ML Time Machine over network drive to Leopard HFS+ fails

Hello,

I am trying to use Time Machine with Mountain Lion on my MBP. I wish to back up to a WD My Book Studio attached to my Powermac G5 running Leopard.

The WD drive is formatted as Mac OS Ext (Journalled). The G5 is happy to use the WD drive with Time Machine and it works well.

I have shared the WD drive over the network and can see it in ML, I am signed in to the G5 over the network and appear to have full access.

I attempted to add the WD drive to my ML Time Machine by opening Terminal and typing:

'sudo tmutil setdestination /volumes/WDDRIVENAME'

I then entered my ML password but was returned with the error:

/volumes/WDDRIVENAME: Incompatible file system type: smbfs (error 45)

The backup destination could not be set.

Does anybody have any advice?

I don't understand why Leopard is able to use the WD drive and ML isn't.

Many thanks.