Restart should be able to eject the disks.

If not, then you eject the disks (dragging them to the trash or hi-lighting them and press Apple-E {for both}, Apple-1 {for the internal drive} or Apple-2 {for the external drive}) and then turn it off through the switch in the back.

tanaquil said:

Anyway, if you have a machine which lacks a Shut Down command, and you turn the machine off and on with the disk still in it, it will try to start up from whatever disk is inserted (whether in an external or an internal drive). If your machine has a Shut Down command, it SHOULD eject all disks when you choose Shut Down.

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Never shut down with a disk inside. Shutting down the switch in the back sends a momentary spike through the system, which is often and mostly harmless. But this spike does travel to the floppy drives and to its head(s) and motors. Though the motors will make the drive like it's giving one last small spasm, it is the spike to the head that can cause corruption to the floppy disk over time.