Sydex TeleDisk - "The Diskette Fax Machine" was an MS-DOS utility for encoding floppy disks that allowed non-MS-DOS standard disks to be read, archived or transferred and then recreated on another machine.
Teledisk is a program which reads a floppy diskette into an image file on the PC, and can recreate a nearly exact copy of the floppy at a later date from that image file. For many years, Teledisk was the only program like this which could handle non-PC formats, and it became the primary method by which people preserved and exchanged system disks for classic computers.
Originally sold as a shareware product by a company called Sydex, all rights to Teledisk were later sold to NTI, who no longer supports it or offers it for sale. This puts Teledisk into a state of limbo, as you cannot legally obtain or use later NTI versions, and the Sydex shareware versions cannot be used beyond the evaluation period without a purchased license, which you currently cannot buy.
If you have a very old PC (MS-DOS/Windows 3.x/pre Pentium) you can download TeleDisk from here: http://www.classiccmp.org/dunfield/img47321/teledisk.htm