You can connect two kinds of disk drives to the Apple IIGS through the disk drive port: 3.5-inch drives and 5.25-inch drives. When connecting more than one drive, you use a technique called daisy-chaining: You connect the first drive directly to the disk drive port on the computer, then you connect the second drive to the first drive, and so on.
You can daisy-chain as many as four drives to the disk drive port, but the following limits apply:
¥ only two Apple 3.5 Drives
¥ only two Apple 5.25 Drives
If you're daisy-chaining drives of both types, the Apple 3.5 Drives must be connected before the Apple 5.25 Drives.
UniDisk drives: Although the Apple 3.5 Drive is the recommended 3.5-inch disk drive, you can also use Apple UniDisk 3.5-inch drives with the Apple IIGS. But there are additional constraints when using UniDisk drives. Before you connect a UniDisk drive, read "UniDisk 3.5-Inch Disk Drives" in Appendix A of the Apple IIGS Owner's Reference.
If you don't know whether your 3.5-inch disk drive is an Apple 3.5 Drive or a UniDisk, the easiest way to tell is by color: Apple 3.5 Drives are platinum (the same color as the Apple IIGS), and UniDisk drives are white. The drives also have identifying labels on their undersides.
You can also connect a hard disk to the Apple IIGS, but you do so through a slot inside the computer rather than through the disk drive port. For instructions on connecting hard disks, see Appendix A of the Apple IIGS Owner's Reference.
Chapter 1: Setting Up Your Apple IIGS