Good day,

My server 2012 "bare metal" recovery backups, which are made daily have started failing as of a week ago.  Event log is spitting out this error:

"There was insufficient disk space on volume C: to grow the shadow copy storage for shadow copies of C:. As a result of this failure all shadow copies of volume C: are at risk of being deleted."

Now, to resolve this I manually changed the shadow copy storage limit to 20% of the C: drive volume which should be plenty, and rebooted the server after the change was made.  My C: drive on the server is 120GB, with less than 60GB being used currently, so I fail to see why I'm getting this problem.  Any advice would be appreciated.

Thanks.

are you run Disk Defragmenter on a volume with shadow copies activated?
The System Shadow Copy provider uses a copy-on-write mechanism that operates at a 16-KB block level. This is independent of the file system's cluster allocation unit size. If the file system's cluster size is smaller than 16 KB, the System Shadow Copy provider cannot easily determine that disk defragmentation I/O is different from typical write I/O, and performs a copy-on-write operation. This might cause the Shadow Copy storage area to grow very quickly. If the storage area reaches its user-defined limit, the oldest shadow copies are deleted first.
solution:
If the file system's cluster size is 16 KB or larger, the provider can recognize disk defragmentation I/O and handle it correctly. Microsoft recommends that you use a 16-KB or larger cluster allocation unit size when you format the volume if you plan to defragment volumes that are used for shadow copies of shared folders, .
Fro detail, pls. refer to
https://support.microsoft.com/en-us/kb/312067
Roger