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.