OK, Monkeh16 & tempeteduson...
...you're probably right in that I either should have done a more through job of covering power consumption or not done it at all. So, since I already did it, I went back and studied the issue some more and rewrote more complete findings in that section.
We know from testing many 2.5" drives in the past that they typically idle at 1W or less, and peak at about 2.5W during write/read operation. A reading of the USB 2.0 and 3.0 Specifications indicates that maximum rated current for each port is 500mA and 900mA, respectively, at 5V. This translates to 2.5W per USB 2.0 port. The current is supplied by the 5V standby line in a PC, usually rated for at least 2.5A. However, the specs appear to be recommended ratings for external USB devices. The Intel ATX12V Power Supply Design guide v2.2 mentions, for example, that on wakeup, an external USB device may cause a "peak currents as high as 3.5A lasting no more than 3 seconds", referring probably to spin-up of an external hard drive. It's not clear whether there is current limiting in the port circuitry; non-compliant devices might be able to draw more than 2.5W, with potential higher than normal long-term stress on the USB circuitry in the PC.
In the recent past, some external 2.5" drives were supplied with a USB cable that had two plugs on the PC side to ensure enough current could be drawn safely. As you know from photos above, the cables for these FreeAgent Go drives have just one port per end. Still, the drive in our FreeAgent Go 1TB is rated for 0.75A at +5V or 3.75W, probably the maximum at spin-up. (See the last page for photos and details.)
It's not easy to test the power draw through a USB connection. About the best we could come up with was to monitor the power draw of a laptop while running these USB 2.0 external drives from it. The battery from the notebook was removed altogether to prevent any extra power going to charge the battery.
The result aren't exactly definitive, but it appears that the 640GB and 1TB draw about the same power from the USB line when idle. The 2.6~2.7W AC additional at idle seems inordinately high given that the drives actually pull no more than 1W DC, even if you includes the power lost through the AC/DC conversion of the laptop. During read/write, the power jumps to around 5~6W AC maximum, which again seems too high. In any case, running these (or any) USB-powered drives off a laptop will have some impact on overall run time on battery; many low power laptops draw just 10~15W AC in normal use, so even the minimal additional load of a USB drive in idle could result in a 10~20% reduction in untethered run time.