Netbackup only supports the appliances listed in the document "NetBackup 
4.5 for NDMP: Supported OS and NAS Appliance Information".  That tells you 
for which supported appliances DAR is available.   It was introduced for 
the first time with NBU 4.5 FP3.

Broadly it looks as if appliances that are supported as NDMP V3 systems 
may offer DAR.   Those on V2 will not; I guess maybe it was not in the 
protocol.

You can in the files list try the various environment variables defined in 
the NDMP protocol.  There is one called DIRECT to turn on DAR.  Looks like 
DIRECT=Y may be a thing to try.  Look at the logs to see what is going on. 
 I think in volmgr it is possible to trun up NDMP logging, but I have not 
seen a docmented way to do it.

William D L Brown