Can I read and write to a FAT12 disk with macOS Big Sur ?

Yes, you can read and write to a FAT12 disk with macOS Big Sur. This means you can copy files to and from the disk just like you would with any other storage device. Simply connect the FAT12 disk to your Mac, and you should be able to access it through the Finder and copy files to and from it as needed.

The capability to read and write to the FAT12 file system has been present in macOS for many years and was included in all versions up to and including macOS Big Sur (macOS 11). As far as I know, all versions of macOS released since the introduction of the Mac in 1984 have included support for reading and writing to FAT12, so you should be able to use this file system with any Mac running a relatively recent version of the operating system.

rupbar2 wrote:

I must now try to understand why I can't write to this 3 1/2" hard "floppy" disk.

That is important information. It may depend on the USB adapter being used, or macOS may no longer have any driver support for a floppy disk. It is also possible the magnetic media has degraded where it is no longer able to be used. I ran into this problem with my floppy disk collection many years ago. I even had some brand new disks sitting around to try, but they had errors galore.