Any 400K or 800K disk images cannot be written with the USB add on floppy drives. Only 1.44MB disk images can be written.
Why? Read on...
The diskette format -
There is a chip inside the Macintosh is called the Floppy Disk Controller, this chip talks to the diskette drives using a recording technique called MFM.
There is a chip inside of the Macs also called the Floppy Disk Controller (Aka Integrated Woz Machine, IWM) it uses a recording technique called GCR.
Reading a GCR disk on a MFM drive is like reading a music CD on a record player (uses vinyl records for music, pre-90s thing). It won't work.
Tech info- The Macintosh (GCR format) uses tag bytes on their disks. These were 12 extra bytes written with every 512 byte sector. These identified the sector, letting a disk reconstructor to put your disk back again in case your diskette got blown away. Unfortunately no utility was created. This resulted in a 524 byte sector.
When Apple introduced their SuperDrive, it limited the High Density disks to 1.44MB instead of the normal 1.6MB. This was done to be more compatible with the PC format. Now the SuperDrives could read DOS, ProDOS and Macintosh formatted disks. No tag bytes!
1.44MB floppy disks are in the MFM format.
400K/800K floppy disks are in the GCR format.
What can I do?
Many have submitted the following way to transfer information from a newer Mac to an older Mac.
To get a Macintosh file from a newer Mac to an older Mac:
Get a file in the .hqx or .bin format
Hook the Macs together with a null modem.
Pick up a two Macintosh serial/modem cables and a null modem. My null modem has 25 pins.
Note: For this to work your newer Macintosh must have an older Serial port.
Use a terminal application (MacTerminal or ZTerm on the Macintosh)
Get them to talk to each other without dialing a number for a connection. (half duplex)
You will know it is working as what you type on one screen will appear on the other screen.
Start the file transfer. If you are using the Zmodem protocol, it will start receiving automatically on the other end.
Unfortunately Apple Computer, Inc. has decided to not let us provide any System Software diskettes anymore.
The Mac 512 is a gYounk design & programmed site.