I had a server that had the "Microsoft Exchange Writer" missing.
When typing the command vssadmin (C:\>vssadmin list writers) from a command prompt all the VSS (Volume Shadow Copy) writers would come up besides the "Microsoft Exchange Writer".
If it does not come up in the list... to enable it navigate to the following registry key:
Modify the "Disable Exchange Writer" registry key to a value of 0
After this restart the "Exchange Information Store" service
"vssadmin list writers" should now display the writer correctly
Check the Technet article for more information: http://technet.microsoft.com/en-us/library/bb795725.aspx