Backing up Network Drives with Time Machine

[swooding]

I am able to successfully back up my F drive to a usb drive connected to my airport extreme using Time Machine. I am unable to back up a second usb drive connected to the airport extreme using Time Machine however. I can see both my media drive and my backup drive as shared drives mounted on my desktop, and if i go to choose disk in Time Machine it offers both disks as potential Time Machine disks, but then although my media disk is not on the exclude list it doesn't then back it up.

Any ideas please !

thanks