Looks like Apple wants you to use a powered USB hub.
Uses for the USB port of Time Capsule, AirPort Extreme, AirPort Express
When I had multiple hard drives plugged into my time capsule - they were self powered and I had a non-powered USB hub and it seemed to work but I remember some flakiness and this might be the cause. Anyway I dropped to 1 large usb self powered hard drive and all is well.

Also I did a little digging - it seems if you have a multi-function printer/scanner/fax you cannot use the scanner capabilities from what I've read. If someone has gotten it to work it would be nice to know.