You really should back up your NDS using an appropriate agent.
As long as you have at least two Netware servers, they will by default replicate NDS and thus be somewhat protected against loss of one server. (However trsutee assignments of users/groups to files and directories are local to each server).
As a very awful security measure, it is possible to copy the hidden files using the toolbox-nlm on the server console. However, it is far from trivial to do a restore that way - simply copyiong back wont help completely. Therefore I don't recommend this nor go into details.
To protect against disk crashes, you may want to use RAID 5 - but then again there is also the risk of burglary or fire...