I've got a WiFi router (Asus) and an external HDD, connected to it by USB. The router gives access to the HDD by FTP and SMB protocol. It's working good as external storage for my MacBook Pro, but I want my Time Machine to create its backups on that HDD. Is there a way to do that? Mac OS 10.8.2.
Time Capsule is actually just an AFP host. You can configure any device that serves the AFP protocol to be a Time Machine backup destination.
However, if the device does not support AFP, it takes some work on the client to get TM to use it as a destination.
You can see the results of that question here: Time Machine over SMB/CIFS share?