Most storage drives come in both internal and external versions.
Internal drives are generally faster and less expensive, but external drives are portable.
IDE is found in newer Macs and most PCs because it's the cheapest way to connect drives to the motherboard. This interface can support up to four devices. Some PCs have SCSI built-in, but on most PC's you need to install a SCSI controller card into an expansion slot in order to use SCSI devices. SCSI devices tend to be more expensive because they have dedicated processors inside them to handle data transfer between them and the computer. Whereas, with IDE the computer's processor does most of the work.