Not to be mixed up with the Apple Macintosh Hard Disk 20, the first hard drive manufactured by Apple exclusively for the original Macintosh floppy disk drive port interface, the Apple Hard Disk 20SC was Apple's first SCSI based hard drive for the Apple II family as well as the Macintosh and other third party computers using an industry standard SCSI interface.

Released in September 1986 along with the Apple IIGS (which required an optional SCSI interface card), it debuted over 9 months after the introduction of the Macintosh Plus, the first to include Apple's SCSI interface. It was a welcome addition at considerably faster data transfer rates (up to 1.25 megabytes per second) over its much slower predecessors, the Hard Disk 20 (500 Kilobits per second) and ProFile. In addition, it was the first cross-platform drive offered by Apple and the first hard drive to use the Snow White design language. Notably, it was the only Snow White product to use the Macintosh beige color and one of the few Apple products to be introduced in two different colors at the same time. Since the IIGS was the first Apple product to debut in the new gray color they called Platinum, the 20SC had to both match it and the beige color of the Macintosh Plus, which it is designed to sit perfectly beneath. All Apple products would change to Platinum in 1987 and remain in use for the next 10 years.

The 20SC contained a 3.5" 20MB hard drive, the same size as the Hard Disk 20, but 10 to 15 MB over what had previously been offered by Apple for the II family. The same drive mechanism would also be offered 6 months later as a built-in drive option on the Macintosh II and SE. It had two standard Centronics 50-pin connectors, one for the System and one for daisy-chaining extra SCSI devices and a SCSI ID selection switch. An external terminator was required if it was the only SCSI device connected. The case itself could accommodate a 3.5" or 5.25" full-height hard drive mechanism. Indeed the case design itself would be reused unchanged (in Platinum only) for 3 more models introduced the following year: 40SC, 80SC & 160SC (offering respective MB storage). While the transfer rates were significantly higher due to the faster SCSI bus technology, the actual transfer rate varied from computer to computer thanks to different SCSI implementation based on developing industry standards.