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Microsoft VSS Writer Requirements

A shadow copy is created for each storage group (Exchange Server 2007) or mailbox database (Exchange Server 2010/2013) that you back up when you back up systems using Microsoft Volume Shadow Copy Services (VSS).

To create the shadow copy, a volume shadow copy is created on each volume or mount point that contains the system files, log files, or database files of the storage group. The default initial size of the shadow copy storage area for the VSS is 300 MB. You should therefore have at least 300 MB of free disk space available on each shadow copy storage volume.

The size of the shadow copy storage area may increase when VSS creates multiple shadow copies on the same volume concurrently. In this case, more available free disk space is required to ensure that your backups are successful.

For more information, see Volume Shadow Copy Service Tools and Settings on the Microsoft web site.