Some USB hard drive devices require the use of both ports. I have a Gateway drive that comes with a special cable with two plugs on one end; Green labeled "Power & Data" and Blue labeled "Power", and one on the other. The single plug goes into the drive and two plugs go into the computer. Have not tried it with the eDGe yet, though. On my Toshiba laptop it will work fine with only Green plug.

Update: The Gateway external hard drive required the use of the USB y-plug without having it plugged into A/C.
The drive particulars are:
Power: 5V @ 1A
Model #: PMD01120H
Part #: 2527961R
5400RPM
8MB Cache
120GB
Though Google search seems it has been discontinued...