Whenever I've added a new drive to my Time Machine backup set of two, I have a hard time getting a remote iMac to see the drive.
I can even mount the drive in the Finder, see it's content, and Time Machine itself still doesn't see it.
I think I found the answer, something I missed previously in Apple's instructions. I've added bold and one comment in square brackets.
Use a network-based disk
If your backup disk is on a network, the network server must use Apple File Protocol (AFP) file sharing. Both your Mac and the networked backup disk should have OS X v10.5.6 or later. The AFP disk must also be "mounted" (available to your Mac) when you set it as your backup disk in Time Machine preferences.
From the Finder, choose Go > Connect to Server.
Enter the server address, or your AirPort Extreme Base Station's LAN IP address (by default 10.0.1.1).
Enter the username and password required to access the network disk(s).
Select the desired disk(s) from the available volumes. [in the Finder]
Select the volume from Time Machine.
After you select the network disk in Time Machine preferences as a backup disk, Time Machine automatically connects to it when it’s time to backup or restore your data.