The Apple Hard Disk 20SC is 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.
The 20SC originally contained a half height 5.25″ Seagate ST-225N 20MB SCSI hard drive, but was later manufactured with a full-height 3.5″ MiniScribe 8425SA 20MB SCSI hard drive. The latter drive was the same size as the drive inside the Macintosh 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 additional 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 would be reused unchanged (in Platinum only) for 3 more models introduced the following year: 40SC, 80SC & 160SC (offering respective Megabytes of 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.[7]
Video:
Here’s the actual item in my museum
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The Apple HD 20SC basics:
Color: Beige | HDD: 20 MB