Formatted empty disk.
When using my first Mac I only had the internal floppy, but did have 4Mb of memory and System 6. So a RAM disk was a sensible way of doing things, I kept my project on the system disk, but after I booted up I copied the applications I was going to use into the ramdisk. Lots of system resources get loaded on demand, including needing a swap in of the system disk, ironically, just to read the hour-glass cursor.
A major advance of the Mac over CPM/DOS/Windows is that removeable media are referred to by the media itself, not the drive they happen to be in. Consider using A: or B: on DOS to get at different floppies, on the mac you can have more floppies available than physical drives, if you eject a floppy and put it back in a different drive the system doesn't care.
Presumably most people will say "What's 'B:' ?" :
