Hello,
Stan, in the method you describe refers to NDMP to NDMP backups, i believe 
you said you have the NDMP tape server licence, is that accurate ? are the 
devices on SN configured as NDMP ? Can you take out all other sn attribute 
from the client resource but the one you want  to backup to and try again 
? What version of NDMP  ?
Can you turn on NDMP debug on the Device and the group that it backup to ? 
 Is your pools, device allocation exclude NDMP devices from the server for 
the group  for NDMP ?
What about zoning, how can NDMP write to devices not zoned to the NDMP 
appliance ? i am not a zoning expert however, backup servers devices 
should not be zoned on the NDMP appliance if that is the case ?
If you are using the over the network method make sure the cmd has correct 
IP for the SN in the cmd line.
Here is the sample cmd  that works with Netapp
nsrndmp_save -T dump -M -P 10.10.0.170 ( IP should be of the server or SN)
HTH

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