Same problem
I'm having the same problem as the original poster. I have a USB drive plugged into my Time Capsule, and although that drive shows up in the Finder, I can't open it and list its contents. It also shows up in the AirPort utility.
Similarly, I can't open the Time Capsule's drive and look into its contents.
When I try, I just get the progress wheel spinning around in the lower right of the finder, and nothing happens.
I have just pulled the USB drive out of the Time Capsule, and plugged it directly into my MacBook Pro. It seems to mount fine there.
I have tried powering down the Time Capsule and restarting it, with no results.
What's odd is that I was able to open the USB drive last night. I was reading an enormous set of mp3s off it and into iTunes, but this morning I couldn't get access to it, and iTunes was hanging. Since then, nothing I do has allowed me to get into the drive.
On the Mac discussion formats at apple.com I see a couple of other postings about what looks like the same problem. They report that the problem is transient, so the discussion has been closed.