A SCSI connector looks somehting like a parallel port connect, big, bulky.
An SATA connector is much smaller and thinner, vaguely like USB, but it will
nit fit in a USB hole.
SATA is mostly for inside a PC, althouigh I have heard recently of external
SATA. SCSI works inside or outside. Ultimately, SCSI can be faster, but is
usually more expensive.
Have you considered USB 2.0 for an external hard drive? In theory it is
slower than SCSI or SATA, but in practice the interface may not be what is
limiting file transfers.
By the way, check the history of IOMEGA drives (ZIP, JAZ, Peerless). Except
for ZIP, which is still sold, their removeable media drives tend to become
obsolete very fast, and their price/Gig tend to be fairly high.