USB and external hard drives

I am looking at getting an external USB hard drive. I know they come in two main flavours 2.5" and 3.5". I would like to have one that can run without an external power source (i.e. USB powered). Which is where I wonder if someone can clarify things for me. As far as I can make out, all external 3.5" drives require an external power source, whereas all external 2.5" drives seem to be USB powered. Is this the case?

My WAG is that a power hungry 2.5" drive will not work without external power. I have been looking at the spec for the new 5200 rpm Seagate Momentus. It quotes a startup current of 1.0 amp max. Comparing this with the wikipedia article on USB (which quotes 500 mA as deliverable on a given bus segment), it would seem incompatible. IANA expert in matters of hard drives or USB, however, so I wonder if anyone more knowledgeable can confirm or deny.

Generally that is the case, yes, but even some of the 2.5" models require external power.

Seems to be the case that the power requirements of a 2.5" hard drive are teetering right on the edge of what can be supplied out of USB (500ma IIRC) - We have some Fujitsu Handydrives here at work and they come with a bonding cable that allows them to draw power from two different computers (or ports on two different roots, if you have them). My experience is that these devices work just fine if they are the only item plugged into any USB port, but expecting them to coexist with other devices often leads to disappointment. If only they wouldn’t equip the bloody things with three high-intensity blue LEDs, that would save ~100mA right from the start.

I’m not aware of any bus-powered 3.5" hard drives - I don’t think it is possible.