Backup fails every other day - Windows Server 2012 R2 Essentials

We have a server with Windows Server 2012 R2 Essentials installed.   Backup is set up using the Windows Server Essentials Dashboard to back up to a USB drive.  Every second day, the backup completes successfully.  On the days in between, backup fails.  This pattern seems absolutely consistent.

On the days when it fails, the Application Event Log contains an error 517 from source "Backup" with detail "The backup operation that started at '?2014?-?11?-?15T01:00:09.561288500Z' has failed with following error code '0x800700EA' (More data is available.). Please review the event details for a solution, and then rerun the backup operation once the issue is resolved."

The above error is logged around 2 minutes after the start of the backup.  No other errors appear in the Application Event Log.  

Backup log files are created in %windir%\logs\windowsserverbackup on the days when the backup completes successfully.  Nothing is created in there on the days when it fails.

Any ideas as to what's going on here, and how it might be fixed, please?
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noxchoGlobal Support CoordinatorCommented:
Are there any other events generated before and after this failure message?
wakatashiAuthor Commented:
Only Informational events - no warnings or errors.   Shadow Copy starts, and it backs up the WSUS and Sophos SQL databases without problems.  Then there's an Event 8220 (VSS) "Ran out of time while deleting files", which I understand is normal (and this event appears even when the backup works).  Then an Event 2230 (MSMQ) "A backup (components: registry, sysq, mapping, queue, storage) was completed successfully".  Then the error.
noxchoGlobal Support CoordinatorCommented:
Ok, then another question. Does this happen if you do the backups to internal fixed drive? What is different when it works and when it does not work? Is the USB drive rotated?
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wakatashiAuthor Commented:
I haven't got a spare internal drive to back up to, I'm afraid, since this backup job backs up everything on the server.  But absolutely nothing's different between days - that's what so strange!  At the moment, there's a single USB drive that stays connected.  And the pattern's absolutely rock solid.  One day it works, the next it doesn't, then next it works, the next it doesn't.
noxchoGlobal Support CoordinatorCommented:
How long does it take to perform a backup? Could it be that previously started backup is not ready yet and thus the second run fails?
wakatashiAuthor Commented:
When the backup works, it takes about an hour and a half to complete.  The backups are scheduled to start at 1am.  So definitely no overlap.
wakatashiAuthor Commented:
More info: Looked in the Event Viewer under Applications and Services Logs | Microsoft | Windows | Backup | Operational.  I really thought I was going to find something useful in there.

1:00:08am: Event ID 1 - "The backup operation has started."

1:02:38am: Event ID 5 - "The backup operation that started at '‎2014‎-‎11‎-‎17T01:00:08.940451400Z' has failed with following error code '0x800700EA' (More data is available.). Please review the event details for a solution, and then rerun the backup operation once the issue is resolved."

1:02:38am: Event ID 14 - "The backup operation has completed."
wakatashiAuthor Commented:
After installing the big November 2014 update rollup, the backup doesn't fail, it just says "Completed with warnings" every other day instead.    If I look at the failed files, they were as follows:

Error in backup of E:\$Extend\$RmMetadata\$TxfLog\ during write: Error [0x80070005] Access is denied.
Error in backup of E:\$Extend\$RmMetadata\$TxfLog\$TxfLog.blf during write: Error [0x80070005] Access is denied.

I've configured the server to ignore these files, as they're not necessary for a bare-metal restore.  See the following for the info I used.

After doing this, backup is consistently reporting successful completion.

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wakatashiAuthor Commented:
Found a solution myself.
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