Backup failed with following error code '0x80780119'

[admin]

Windows 2012 R2 Backups fail with “There is not enough disk space to create the volume shadow copy...”

An Application Event log entry is created with this useless information (see the attachment for the bigger picture):

"The backup operation that started at '‎DATETIME.883000000Z' has failed because the Volume Shadow Copy Service operation to create a shadow copy of the volumes being backed up failed with following error code '0x80780119'."

Here is our configuration:

- New 2012 R2 Server installation, 64GB RAM, RAID50 SAS
- C: Drive = 200GB (160GB free space)
- D: Drive = 5TB (4.3TB free space)
- E: Drive = 2TB (2TB free space)

Destination is a 2TB USB3 drive (unused/no data) configured for use in Windows Backup.

This is part of a “hybrid” backup solution we are trying to put into place.

File-level Backups are done with Backup Exec 15.
We want a simple Windows Backup that includes Bare Metal Recovery and Hyper-V VMs.

Google tells me that this is a common error with Windows 7/8 computers and there seems to be myriad of complex work-arounds which we are hesitant to “try” as the Server is now in production.

[robinhood]

What is the amount of free disk space on the System Reserved partition? The System Reserved partition is commoly the root cause of the issue.

[admin]

Recovery Partition = 300MB (free space is 300MB)
EFI System Partition = 100MB (free space 100MB)

No other partitions exist other than the actual "partitions" (C:, D:, E: etc.)
I am "presuming" that the "EFI System Partition" is the "System Reserved" you are questioning?

[admin]

Update:
After "excluding" Bare Metal Recovery (which excludes the "system partitions" which seem questionable) the Backups run just fine.
Still, it defeats our purpose of using Windows Backup as a "simple" Bare Metal Recovery - a shame as Windows Backup has been pretty accurate and trustworthy at most other customer sites.
Seems to be a shortage of space on the aforementioned partitions (Windows Explorer incorrectly reports amount of available free space on those partitions).
Again, I am hesitant at the moment to try edit these partitions - would you?