Following types of errors are seen in session messages:
[Major] From: VBDA@hostname "Z:" Time: 5/22/2016 1:18:58 AM
[81:78] GLOBALROOT\Device\HarddiskVolumeShadowCopy1991\WindowsImageBackup\hostnaem\Backup 2016-05-20 150016\a15ca12c-c1f2-11e4-8cb8-806e6f6e6963.vhd
Cannot read 261832 bytes at offset 31735345344(:1): ([2] The system cannot find the file specified. )
[Minor] From: VBDA@hostname "I" Time: 5/14/2016 8:18:47 PM
[81:75] \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy3058\Common\pd\DATA\June 2012\
Cannot traverse: ([2] The system cannot find the file specified. ) => not backed up.
Those are signs of deleted VSS snapshot. Usually this happens when dedicated VSS shadowstorage space is not sufficient for the amount of volume changes during the session. Windows event logs should contain exact reason eg:
.Error 22/05/2016 1:18:58 AM volsnap 25 None The shadow copies of volume Z: were deleted because the shadow copy storage could not grow in time. Consider reducing the IO load on the system or choose a shadow copy storage volume that is not being shadow copied.