wd 2,5″ usbdisk on Pi2

Hi,

i am trying to get a WD 2,5” usb disk working on my Pi2.
the light on the disk is on when i connect the disk but i cant find the disk in the Pi2
the only thing in the usb ports are the dongle for the remote control and this WD 2.5″ usbdisk

so can this be a powerproblem or am i doing something wrong?

thanks