I heard from local experts that the only possible way to save the NDS
is using a tape device.

We have 6.5 netware and in case of problems I wish I had a copy of the
users, rights, directory system, etc which I can restore easily in case
of system crash

tapes are obsolote and often damaged. The netware 6.5 server has DVD
rewriter and USB, is it possible somehow to save these information onto
DVD or a pendrive?

I cannot beleive the only solutionm is the old tape method....