The hard disk interface defines the physical and logical means by which the hard disk connects to the PC. In the 1980s, the most popular disk interfaces were ST506/412 and ESDI, which are now obsolete. These old drives use two ribbon cables (a 20-pin data cable and a 34-pin control cable) versus the single ribbon cable used by modern drives. Finding one of these old dual-cable drives in a PC by itself establishes that that computer is too old to be upgraded economically. A modern PC uses one or both of the following hard disk interfaces:
SCSI disks are seldom used in desktop PCs because they cost more than IDE disks with similar capacity and performance. For example, if an IDE hard disk costs $90, a similar model with a SCSI interface may cost $175. In addition to the higher cost of the drives themselves, using a SCSI disk requires installing a SCSI host adapter, which may add $50 to $300 to system cost. However, spending extra money on SCSI may increase overall system performance more than spending the same sum on a faster processor or a high-end video card, so don't rule SCSI out. In our experience, even relatively slow SCSI hard disks outperform fast IDE disks under load, particularly under multitasking operating systems like Windows NT/2000/XP and Linux.