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Data Interface Connector
Modern hard disk drives use one of two interfaces: IDE (ATA) and its variants, or SCSI. You can tell immediately by looking at the back of the hard disk which interface is being used by the drive:
The connectors on hard disk drives are generally in the form of a 2xN rectangular grid of pins (where N is 20, 25, 34 or 40 depending on the interface). Older ST-506 (also called MFM, RLL) and ESDI hard disks used two data connectors, one 34 pins and the other 20 pins. These connectors were often not in the form of pins but rather card edge connectors, such as those used by ISA expansion cards. Some SCSI connectors may have different shapes, especially older ones.
While most current SCSI interface connectors are keyed to prevent incorrect insertion (because they are D-shaped), this is not always the case for other interfaces. For this reason, it is important to make sure that the cable is oriented the correct way before plugging it in. The cable has a red stripe to indicate wire #1 and the hard disk uses markers of one form or another to indicate the matching pin #1.