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. A single SCSI controller lets you daisy chain up to seven devices, and most controllers have connectors for internal and external devices. Daisy chaining several devices can sometimes slow data transfer.
Internal drives are almost always faster than external drives.
If you want the portability of an external drive, the interface options are parallel port, SCSI, USB and PC Card. A parallel port is the slowest option, and it only supports two devices. You can daisy chain up to 63 devices on FireWire. Like USB, FireWire supports hot swapping which means you can add or remove a drive while the computer is running. ADS Technologies has a kit that will adapt any internal IDE device into an external FireWire device through a special PCI card.