Macintosh Floppy Emu web site at Big Mess o' Wires
"Love old Macintosh computers? Floppy Emu is a hardware floppy disk drive emulator for vintage Macs. It uses an SD memory card and custom hardware to mimic a 400K, 800K, or 1.4MB 3.5 inch disk drive and floppy disk. It plugs into the Mac's external or internal floppy connector, and behaves exactly like a real disk drive, requiring no special software on the Mac.
"Floppy Emu is perfect for setup or troubleshooting of a Mac without a hard drive or a working floppy drive or OS. Just plug in the Floppy Emu, and you're booting up in seconds. Keep it as a permanent solution, or use System 7 installer disk images to do a new hard drive installation. The hardware is also great for moving files between vintage Macs, or Macs and PCs with an SD card reader.