When you say you bought the "disk writer", do you mean an external 3 1/2" diskette drive? I don't understand what you are saying about updating the "D1 to program for writing disks". Are you saying that Embird want you to apply some software directly to the D1 machine?
I have had a Designer D1 with a floppy for about 10 years. There is a proprietary directory structure for floppies that is required in addition to the correct file format. You can't just drag & drop the file to the disk. As far as I know, you need a copy of Organizer. When the Designer SEs were released, a copy of this software was bundled with them.
3D Embroidery Software (3DQuickfont, 3D Organizer). part #412843126. I lost my CD a couple of years ago and it was about $20 to replace. You do not need the entire 3D Embroidery suite, just organizer. If you can find an old version of the 3D embroidery suite, you do not need the dongle to install Organizer. Having this will allow you to send 1 design to a floppy.