Generally speaking a hard drive is a hard drive no matter what packaging it has. The main things you have to watch out for are the interface (SATA, PATA, SCSI, SAS, Fiber-SCSI, etc.) Most external storage systems sold for home or small office use now a days are simply SATA drives in some sort of housing. Logically YES you should be able to pull the drive our of the unit and use it in another.
Thanks rch1231 it seems sense to buy an external drive and strip it because to buy a 7200 rpm sata drive bare costs from £110 upwards to £150 or so and I can get a WD external 7200 drive for £95 delivered...:)