If you're not locked in to Serial, consider parallel.
A number of outfits, including MicroSolutions ("Backpack") made parallel
port external drives, including 1.4M, 2.8M, and CD-ROMs.
Floptical drives are SCSI, and will also read/write 1.4M. There are a
variety of parallel to SCSI solutions, such as Adaptec
(Trantor) "Mini-SCSI".