The 6300 only support DDOS 6.x and above.  It has new hardware that requires the new code.  I would also recommend making sure you are running DDOS 6.0.1.0 code also.

And if we do not upgrade all the clients, what issues could we find? Bad deduplication?

Thanks

If you use ddboost, ddboost module on client side might not be compatible meaning you might not have client direct, but rather save will use storage node as write through.

If you don't upgrade the clients you may not be able to back them up.  The boost compatibility is very important to make sure you are in compliance with it for the best experience. 

Do you recommend to upgrade the DD firmware to the last version 6.0.1.10?

We have to upgrade Networker to 8.2.4.x in order to be compatible with DD OS 6.0, which hotfix is recommended?

The last one 8.2.4.6?

Thansk

Yes we recommend that you upgrade.  If you are already running DDOS 6.0.x the upgrade will not impact your compatibility. DDOS 6.0.1.10 is a critical upgrade and should be done as soon as possible.  

Yes, your DD6300 is already on DDOS 6.something - please update to 6.0.1.10 ASAP.

The upgrade will make difference to compatibility inside the same DDOS code family (6.0.x.x)