Alternate Disk

The Alternate Disk (or Unknown Disk) was an invisible holding space for files on Macs with only one floppy disk drive.

Description
The idea was that the Alternate Disk would hold files in memory while data was being transferred from one disk to another one.

Apple, at the last minute, figured out a much better way of moving files across disks. Although the Alternate Disk was mentioned in the original Macintosh instruction manuals, it never appeared in practice.