I haven't a digital recorder (DAT aor Computer) so I haven't tried this yet, but as I recollect from the FD-4 manual, you can transfer the data two tracks at a time to a digital format (DAT, hard drrive, CD-R, etc.). They cannot be played back from the system you downloaded because it will be stored in the Fostex format so it must be uploaded to the Fostex again. This data transfer is just to make space available on your recording media for teh FD-4. Check the manual, as I said i haven't attempted this. Now if your talking about send a digital adio out in stereo to another device for mastering purposes I can't see what the problem would be other than you might have to assign what type of signal (audio vs. data) is being sent to the optical digital outs. The manual isn't the easiest to read, but it does seem to pretty good about taking you step by step through the process.
Peace, Jim