50 years is a long time by archival standards. Most media/drive
manufacturers guarantee 30 years under nominal conditions. You'll have
to store your masters under optimal conditions and check them regularly
(read them every couple of years, check error rate, copy if needed
etc.) to go beyond that. Which raises the question where you'll find
the equipment (spares, service) a couple decades down the road. Example
from my perspective: If you have DLTtape III tapes lying around from
the early nineties, you'd better do something about them now since
Quantum EOL'd the DLT8000, which is the last generation that will read
those tapes. Service will be available for another 5 years. That's 20
of the 50 years.
Basically you'll have to copy the data to state of the art media about
every decade. Don't try to store spare drives for the future, that
doesn't usually work - electromechanical devices age when they're not
in use too.
There have been numerous stories about the problems NASA has retrieving
old data recordings. Your project will face the same. Fortunately 20 TB
isn't a big deal any more and will be less so in the future. The front
end doesn't really matter, but the archive will need a lot of thought
and care. Think what the state of the art was 50 years ago.