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Erm... :confused: Where do you see this terminology used?

External hard drives are just internal hard drives in enclosures. I know that's verging on a G.W. Bushism, but I guess maybe that's not clear.

The drives in 3.5" enclosures are the same SATA or PATA drives that desktops use.

The drives in 2.5" enclosures are the same SATA or PATA drives that laptops use.

A few things that are notable...

1) Cheap external drives are much more likely to use PATA. This is fine, unless you wish to be able to swap the drive back and forth between an internal slot. I.E. if you get a PATA enclosure, you will not be able to put a drive from any current Mac into it.

2) Some enclosures require external power and others don't. There are pros and cons either way -- external power makes sure that supply to the drive is consistent and also makes sure that bus power on the computer isn't fully tapped -- it's probably better if the drive is sitting on the desktop. If you move it around a lot, bus power may be more convenient.

3) Firewire vs. USB -- there are lots of threads about this. FW is more expensive but also considerably faster in many situations.

Other than those considerations and size/looks, you just pick the drive you want and away you go.
 
in short, portable hdds can be carried anywhere without a power brick. less space than desktop hdds, but more convenient if you're on the go and can deal with say 80-160gb. also only runs at 5400RPM as opposed to desktop's 7200RPM+. not really noticeable when you're just transferring stuff. generally only usb (2.0)

desktop hdds hold much more (250GB to over TB), can have more options as to connecting to computer (firewire, usb, and/or eSata). also runs beyond 5400RPM (generally 7200RPM) power brick means you need wall outlet. powerful, fast, less convenient.
 
The desktop ones are bigger (in case design most of the time). Portable external hard drives are small in form factor an do not need an external power source for easy, well, portability.
 
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