One of the apple fans (&haters). I have a couple of macs at home and have done a back up to a HDD via USB, now that HDD is plugged into to the Airport wifi router and mounted as a drive on the back end of that. It shows as a shared drive on the network. Now when I try to get it to do the back ups over the wifi I can set up the time machine to navigate to that drive, but when it comes to the back up and starts a brand new one and can’t find the initial back up.

Any ideas?

Stick to doing it by USB? I prefer electively backing up, as I then put the HDD in the safe, just in case of burglary/fire.