How to Fix VolSnap 25 Error The shadow copies of volume C: were deleted

If you encounter a VolSnap 25 error with this type of message or similar:

The shadow copies of volume C: 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.

The issue has to do with a limited diff area space to work with. There are several possibilities:

1. The disk referenced in the error (C:) has no more free space left. You need to have enough space for the duration of the backup to buffer all changed blocks that occur during backups. For lengthy backups on busy servers this could be a considerable amount of free disk space needed.
2. Microsoft has a silly default limit of just 300MB. You can lift that by following the instructions and screenshots here: //backupchain.com/Cannot-Find-anymore-diff-area-candidates-for-volume-Volume-shadow-copy-service-error-troubleshooting.html
   1. open a command prompt as admin as enter: vssadmin resize shadowstorage /for=c: /on=c: /maxsize=unbounded
   2. do this for all drives
   3. on regular PCs, turn off System Restore in Computer->Properties
   4. on Windows Server, turn off all schedules shown in vssuirun.exe for all drives unless you absolutely need automatic shadows for file version restore
3. Another possibility is your server or PC is cluttered with VSS shadows. Delete them all, see instructions: //backupchain.com/i/how-to-delete-vss-shadows-you-cant-delete-outside-of-your-allowed-context and //backupchain.com/i/how-to-delete-all-vss-shadows-and-orphaned-shadows

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