Reference this previous post relating to the same issue:
While this worked initially the same thing has happened again, as in many of the VSS writers are now missing. We feel it is shadowProtect that is interfering with them but cannot find proof.
We would like to keep ShadowProtect installed in case a historical restore is required. The server is running v188.8.131.5257.
Now we have Exchange SP3 installed is there a way to get the writer's functional again once and for all?