How to add a WD My Book Essential 2TB to a Time Capsule?

I bought a WD My Book Essential 2TB External Hard Drive and formatted it Mac OS Extended (Journaled) (and guid partition type). Connected directly to my Mac it works perfectly. But when I coonect it to the Time Capsule’s USB port it does not seem to be recognised. In the Airport Utility it does not show up under Disks. I dit restart the TC but nothing seems to happen. I seem to be blocked here for now. Any leads? My ultimate usage would be to extend the capacity of my TC. I used both AU v5.6 and v6.0

It could be a problem with the TC USB port, have you tried to connect another external hard drive?

Thanks for that reply - the outage a few weeks ago cleared my profile it seems - hence the late reply. 

I did try to connect a different external harddive - i tried a non-powered iOmega and it was perfectly recognized by the TC. 

I am a bit lost - why does my TC not simply recognize my WD external harddrive?