You are partly correct Tousif.
Density in NBU (hcart, hcart2 etc ...) has no technical meaning, it is just a label. The rule is densityX tape will only be allowed in densityX drive. The one exception is qcart which must never be used apart from with 1/4" drives, which I doubt anyone has now. Reason being that for qcart, NBU writes in fixed blocks, which is incompatible with other drives.
Aside of that, the denisty has no special meaning, and will not affect performance or reduce capacity, and any density (apart from qcart) can be used with any drive. If you have a TLD robot, it might not allow for exmple 4mm density as (if I recall correctly) 4mm drives were not available in TLD type robots - I could be wrong on that point though.
This is absolutly correct:
The NBU is application send the data via operating system to destination Storage (Disk/Tape).
This might not be ...
If Media server Operating System support hardware then I don't think NBU have any issue with that.
In theory, an older version NBU 'might' work with a newer drive - updating the device mappings files would be a good start (device mappings file allows NBU to be easily updated with new hardware as it is released).
There could be issues however with 'unsupported drives' - although NBU doesn't write directly to tape (it goes via the OS as you mentioned) we do interact, for example, sending scsi commands, so it's not impossible that some way we 'interact' with older drives is a bit different to what newer drives require.
