I'm liking the small (cigarette pack size) external HDs that are available... many of which are powered by usb only.

Some of them, though, require use of the A/C power adapter.

I'm shopping for one between 640GB and 1TB.

I'm using a 1TB model that a friend loaned me but 1) it is A/C powered, and 2) It's owner manual specifically says that it should NOT be used as a portable.

So what's the big difference between a) what they call "portable" or not, and b) ones that need an A/C adapter vs. ones that are powered by USB only.

Thanks!

Portable drives are tend to be USB bus powered and more rugged in design.
At present they are around 500 Gb max size as they use smaller 2.5 inch drives normally used in laptops.

The large drives, as you have seen, require separate power and use the larger 3.5 inch desktop drives. Also their maximum capacity is around 2 Tb.

I want and need portable. I'm seeing ones advertised as "portable" up to 1TB (like this Seagate Model)

So where is the limitation (and differences) as far as what needs A/C power?

Actually the one you've linked to needs extra power.
The way I look at it, if it's USB powered then it's portable, otherwise not.
So this one would meet that rule: Western Digital - My Passport Essential SE 1TB External USB 2.0 Portable Hard Drive - Black - WDBABM0010BBK-NESN

It;'s all down to the drives employed.
The Seagate is using a larger and faster desktop drive, whereas the WD is a smaller and slower laptop drive.

Thanks, Curls. I'm looking for maximum size that does not need a/c power. Speed is not so important...

Thanks again, Ben. I just bought the WD 1TB $129 External drive.

Blows my mind at how much hardware prices have come down. The late external that I bought was a 320 GB model, 3 years ago. I paid about $250 for it.

Now I get 1TB for $129... what a deal!