We had the same problem, media write errors. Seems to happen with long running 
jobs most of the time. We ugpraded the Data domain code and OST code which 
increased the timeouts to 3hrs from data domain. We also noticed are long 
running jobs in which most of the 84 errors were coming from were Window 2008 
or Windows 2008 R2. there is a known hotfix from Symantec on this issue.
Here is the link for this:
http://www.symantec.com/business/support/index?page=content&id=TECH150081

We upgraded to the following:
DD670
5.0.1.7-250021
OST plugin 2.3.2.0
NBU - 7.0.1

Once we upgraded to the current versions above and applied the hotfixes we now 
get a media write error 84, maybe once a week and at times we will go for weeks 
with not issues. I am to believe the OST plugin and/or NBU 7.0.1 still has a 
bug but neither vendor has a compelte fix.

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