I found an old Mac Plus, and when I turned it on, it had a flashing image of a floppy drive on the screen. Does that mean I have to have a OS floppy? Or can it only be run using an empty one?
The Macintosh Plus requires a boot disk to start up; you'll need some way of writing an 800K floppy disk. This thread has some information, but it seems at the time you needed an old Mac to create the disk.