o.k. What you call a "tape reader" actually is called a "device" in NW. If you mix terminologies, how do you expect someone to understand you?
Of course, if you want the device to be controlled by NDMP, you must configure it as NDMP device. The actual backup core command would be "nsrndmp_save -T dump ...."
Are sure that you have set the "-" in front of the "T"?
If you want to backup to a NON-NDMP device, you must add the option "-M" to the command above. But this means that the data must be sent via the network as you should not be able to define such device at an NDMP filer. I doubt that this is even possible.
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Coming back now to your original question: "Why does nsrmon fail". Actually, i do not know yet.
If enabled, the NW server monitors devices (nsrmon) and restarts the appropriate daemon, if neccessary. For that purpose, the two NW client programs have to communicate. However, on a NDMP filer (NDMP device), no such one exists. I do not know what nsrmon will do in this case. It will probably launch a nsrmmd 'helper' at the NW server. And yes, it will also monitor those.