Apple sold the SC20 about that time. A 20mb SCSI hard drive. Apples first SCSI card for the Apple II was what was later nicknamed the Rev C SCSI card. It wasn't that fast but it made the IIgs seem like a whole new computer. Later they made the Apple Hi-Speed SCSI card which did DMA. Others made S CSI cards too. THe CMS SCSI card wasn't one I liked, some versions had a huge amount of jumpers. The RamFast SCSI card was my favorite and the fastest SCSI card.I'm curious why it shows in the manual what looks like a HD20SC attached when there was no scsi port on the IIGS.
Also SCSI lets you play with CD drives, Magneto-Optical drives, scanners, Zip drives etc.
There were also IDE cards. Applied Engineering made a card - hard drive combination called the Vulcan. Nice but the ROM on the card was pretty much hard coded to the drive so you had to buy your drive from AE. In the last few years there's been a few cards that could use compact flash cards or ide drives though.
Wayne