For the latest information please consult the KB 502616
Upgrading the Data Domain Operating System (DD OS) on a Data Domain system affects more than the Data Domain system itself. It also affects the array of software applications and hardware devices on which your backup and recovery activities rely.
"It may not be compatible with any third-party software or hardware currently in use at your installation and may prevent your Data Domain system from interoperating with those software applications and hardware devices."
For example, if you are using an incompatible version of NetWorker with DDBoost and upgrade to DD OS 6.1, NetWorker will not be able to access the data.
Data Domain maintains an active program of testing software and hardware that are used with Data Domain systems. To make sure that all key components of your Data Domain system installation are compatible with this version of DD OS, you can access Data Domain software compatibility information at this website. When using Data Domain systems running different version of DD OS, please check the Data Domain Operating System 6.1 Administration Guide for Replication version compatibility.