Using a non supported Timemachine volume
Always forget this bit:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Speficially needed if you are using a Linux box, shared by Apple Talk.
From here in this instance, although there are loads of howtos.
http://www.kristijan.org/2011/07/mac-osx-lion-afp-shares/
Is also a useful resource.