And I suppose you changed your previous slave drive as... ?
You say you have a DVD-Rom... Is it master? Slave?
If it's slave you have to change it to master.Otherwiser only Nec-3520, the DVD-rom or none of these will work.
It's impossible to have two master/slaves in the same IDE. Your PC BIOS will find a drive or none.
If that is the problem, just change jumpers: DVD to master and NEC to slave.