They are "Usually" just mini IDE drives but sometimes they have special adapters bolted onto them (can usually be removed to turn it into a normal mini-IDE but then if the new machine also requires an adapter your screwed).
Only once have I seen what appears to be a disk actually built in with a custom connection thus unusable in another machine.
To move it into a desktop is easy as long as you can strip it down to a standard mini-IDE configuration as there are adapters for it, I have one on my desk right now in fact
(It uses one power socket and one IDE connection from the desktop to power the mini IDE through a single connection)