Power supply

The USB specification provides a 5 V (volts) supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.35 V between the +ve and -ve bus power lines.

Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 100 mA up to a maximum of 500 mA. In practice, most ports will deliver the full 500 mA or more before shutting down power, even if the device hasn't requested it or even identified itself. If a (compliant) device requires more power than is available, then it cannot operate until the user changes the network (either by rearranging USB connections or by adding external power) to supply the power required.

If a bus-powered hub is used, the devices downstream may only use a total of four units — 400 mA — of current. This limits compliant bus-powered hubs to 4 ports, among other things. Equipment requiring more than 500 mA, hubs with more than 4 ports and hubs with downstream devices using more than four 100 mA units total must provide their own power. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available.

Typcially, a USB external HD will draw much more than this to power the motor, so don't expect to attach one without adding an external power supply. To verify, just read the label on the device; it should state clearly
the Voltage and Current requirements

edit/
btw: there's also a limit to the number of USB devices attached to a single 'port',
which is 127 (obviously you need some USB Hubs to make this work)
/edit

And very good and useful post, nice one. :-)

A good 2.5" external hard disk enclosure (laptop HD-size), with a hard disk up to 60GB, can run off 2 USB-ports, using its own supplied 2-prong cable.
No external power required in this case.

be sure to read the label on the device -- I ran into this just last night with a
friend and the 1.5amp requirement made it clear, for this device at least,
it was wishfull thinking to use the twin-tail usb cable.

this experience was the motivation for this thread.

A 100GB Seagate notebook USB drive needs external 5 Volts

the CURRENT rating is usually the limiting factor

Yes Jobeard, the additional 5 volt source supplies more current to the USB device

Typically only required for ext USB HDs

I've seen some USB external adapters perform better when they are additionally powerered by an external source

hum; never seen a USB Network adapter with external power