Backup of Mounted Exchange .edb files are completing with partial backups

Hello team,

we are using symantec netbackup 7.5.03 for taking backups of Exchange 2007 mailbox database, here is what we are doing.

1. Take snapshot (point in time copy) of the production exchange databases using our Snapshot application (its an VSS application consistent snapshot).

2. We mount the snapshot to Media server (NBU) and start the media backup of these files.

3. Once the backup is started we are hitting these below issues, 

8:32:33.111 AM: [5580.4416] <2> tar_base::V_vTarMsgW: ERR - failure reading file: C:\3parSnapshot\092413073700\f5a0eab0-24a6-11e3-b540-0022196d93d0\mailboxdatabase5\mdb53.edb (WIN32 21: The device is not ready. )

the backups are completing with partial error.

Is that we are doing something wrong, need urgent help on this issue.

Thanks,

Vinay