I have a network of Mac laptops and Mac Minis. I also have an old G4 running HornWare SharePoints that allows me to use my Drobo as a network share (AFS).
Time Machine does not appear to support backup to a network share and hacks to enable this appear to be fraught with unknown danger as message boards suggest Apple disabled this for a reason.
What is the best way to back-up up Macs over a network?