[H] External drives (USB) and power requirements

[Thane Sherrington]

I notice a lot of external drives (2.5") come with a single headed 
USB cable, yet the drive requires 750 to 1000 mA.  How can a single 
USB plug provide enough power to keep the drive viable over the long term?

(I also notice I see a lot of dead drives like this - the dual 
headed-USB ones seem to last longer.)

T