When you have an AppleTalk network cable connected to the printer or modem port, the port impersonates a card in slot 7not slot 1 or 2 as you would expect. But because the AppleTalk cable plugs into one of the two serial ports, the Control Panel Program won't let you activate the printer port, the modem port, and AppleTalk all at the same time.
When you turn on the Apple IIGS power switch, the first thing the computer does is check its slots for a disk drive controller cardan interface card that controls one or two disk drives. It looks first at slot 7, the highest-numbered slot. If it doesn't find a disk drive controller card there, it looks in slot 6, the next-highest-numbered slot, then slot 5, and so on, until it finds a disk drive controller card. When it finds a disk drive controller card, it checks the disk drive connected to that card for a startup disk. This method of looking for a startup device is called scanning.
Changing the startup drive