Oh, and adding to how old snapshots get deleted which, to you, seems
premature, the growth in usage of the shadow storage by a snapshot may
outstrip how fast the VSS service can write (and enlarge) the snapshot.

https://social.technet.microsoft.com...=windowsbackup

Apparently when shadows get deleted, an event gets logged. I'm not sure
event ID 25 by the VolSnap source is the only one that records deletion
of snapshots.

https://support.unitrends.com/Unitre...icle/000003818

According to that article (a copy of an MS KB article), the
VSS_E_MAXIMUM_NUMBER_OF_SNAPSHOTS_REACHED error means there is some
maximum number of snapshots that can be written to the shadow storage
space.

Since the storage space is reserved and not immediately allocated,
another possibility is there is not currently enough free space
available on the drive to allow growing the size of the SR storage
space. You might have a huge size configured for the SR storage space,
but if there is not enough free space on the drive then the SR storage
space cannot grow which means it has to delete old snapshots to make
room for a new one.

What is the configured max size for the reserved SR storage space?
Multiply that percentage by the size of the drive (partition or volume)
that is getting included in the full backup job. Then check if there is
that much free space, or more, left on the drive. Just upping the max
size of SR storage space will have no effect on the free space available
on the drive. That's probably one of the reasons why SR storage space
is, by default, about 5% of the drive's size but it is recommended you
keep 10%, or more, of the drive's capacity as free space. There needs
to be room for more snapshots, room for the usage to grow for the SR
storage space on an as-needed basis, room for temp files, pagefile, and
yadda yadda to make sure everything keeps running. Getting low on free
disk space, and even before Windows alerts you with its "low on disk
space" alert, causes problems. You could delete programs, install them
on other drives, remove data files to other drives or removable media,
and other cleanup to up the free space, but that just masks and prolongs
the real issue that you need a bigger disk.