Let's go back to step one.
You mention a 40 MB 2.5" SCSI drive and a 20 MB, 3.5" Rodime (proprietary) drive. Do you mean you want to swap platters between those two? Or just that you want to perform this on those two completely separately?
Assuming you mean to do it completely separately, it would be possible. But, for the love of $deity, don't sacrifice a working HD20!!! (I have two dead HD20s that I need to rip open and try to fix, but I would never even consider ripping open a known-good one in an attempt to get either of my broken ones working.)
If the information is really that valuable, pay a dedicated data recovery company to recover it.
