USB connected to Airport Extreme as Time Machine??

Someone told me I could do this so I attached a 4TB drive to my Airport Extreme to use it as a time machine for both my iMac and my Macbook Pro. I then opened Time Machine preferences to attempt to set up it and it showed me three disk options: the current backup on my NAS; another backup on y NAS (the one currently being used for my laptop) and an option "Other Time Capsule". Nothing about the new drive.

So I went into Finder to locate the drive...and can't find it anywhere. It shows up in the Airport utility but nowhere else.

1) How do I get my iMac to connect to it and when I do where will it appear?
2) How do I get Time machine to use it?

Has the drive already been formatted? If not try formatting it as HFS+ extended Journaled then attach it to your Express and see if that helps.

It was pre-formatted for Mac. I went into AIrport Utility and selected "allow setup over WAN" and the router appeared on my sidebar where it hadn't before. Then it showed up fine in Time Machine Prefs. Didn't think that's what that option did, but it worked.

Thanks though...

Glad you got that sorted out. I never would have thought of that. I think none was already set that way when I started using Time Machine so it just worked.