On my mid-2011 iMac running OS X 10.8.2 (Mountain Lion) I was trying to download and copy onto floppy disks (for an old Macintosh) some software downloaded from the internet. I have just purchased and am using a new generic USB 3.5" floppy drive.
I found that I could read such disks via a USB floppy disk drive but that I could not write to them.
It seems that such disk formats have only limited support in more recent versions of OS X.
There is software called MacDisk for Windows. I just ran it in Windows 7 running as my Bootcamp partition accessed via VMWare Fusion. The software is free to test (with some limitations).
I just successfully copied a file via it to a disk and then was able to read it on my iMac.
Hope this helps you, or anyone else looking to do such strange things.