Use this article when DATASTOR Shield backups fail because Windows Volume Shadow Copy (VSS) could not create or grow a snapshot. Related errors: 0x80042308, Volsnap Events 25, 33, and 36.
All commands require an elevated Command Prompt or PowerShell (Run as administrator).
Important: two different limits
The Maximum Shadow Copy Storage size is only a cap. VSS still needs real free space on the storage volume to grow the copy-on-write file while a backup is running.
Example: a 2 TB volume with a 215 GB (10%) cap will still abort the snapshot if the volume itself has only a few gigabytes free.
1. Inspect current state (read-only)
Run these first. They are safe and do not change settings.
vssadmin list shadowstorage vssadmin list shadows vssadmin list writers vssadmin list providers
| Command | What to look for |
|---|---|
list shadowstorage | Used / Allocated / Maximum per volume. Note whether storage is on the same volume as the data (For = On). |
list shadows | Existing snapshots. After an Event 25 purge, this list is often empty for that volume. |
list writers | Every writer should show State: [1] Stable and Last error: No error. A failed writer can fail the next backup even after space is freed. |
2. Recommended size limits
- Do not set production data volumes to
UNBOUNDED. Persistent Previous Versions will consume the drive. - Do not cut the cap so low that a backup snapshot cannot grow.
- Keep 10–15% of the volume free at all times. That free space is what VSS actually writes into during the job.
| Volume size | Suggested maximum | Notes |
|---|---|---|
| Up to 256 GB | 15–20%, or 10–20 GB | Leave at least 15% free on the volume |
| 256 GB – 1 TB | 10–15% | Typical system / mixed-use disks |
| 1 TB and larger | 5–10%, or a fixed GB cap | A 2 TB D: at 10% = ~200 GB max — use a fixed cap if you do not need Previous Versions |
OS volume (C:) | 10–15%, at least 10 GB | Raise the cap if Used is already near Maximum |
3. Resize the cap on the same volume
Replace D: with the volume you are changing. Lowering the max deletes the oldest shadows if current usage is above the new cap.
Fixed size (preferred on large data volumes):
vssadmin resize shadowstorage /For=D: /On=D: /MaxSize=50GB
Percentage of the volume:
vssadmin resize shadowstorage /For=D: /On=D: /MaxSize=10%
No cap — avoid on production data drives:
vssadmin resize shadowstorage /For=D: /On=D: /MaxSize=UNBOUNDED
Valid MaxSize units: KB, MB, GB, TB. Minimum is 1 MB.
If no association exists yet, use add instead of resize:
vssadmin add shadowstorage /For=D: /On=D: /MaxSize=50GB
4. Relocate shadow storage off the data volume
Best fix when the data volume is full or I/O-heavy. Event 25 explicitly recommends a storage volume that is not being shadow-copied.
Requires Windows Server. E: must be a local NTFS volume with spare space, preferably not in the same backup set.
vssadmin add shadowstorage /For=D: /On=E: /MaxSize=50GB vssadmin list shadowstorage
If a D:-on-D: association still exists and you want only E: to hold the diffs:
vssadmin delete shadows /For=D: /All vssadmin delete shadowstorage /For=D: /On=D: vssadmin add shadowstorage /For=D: /On=E: /MaxSize=50GB
5. Delete existing shadows
Caution: These commands remove restore points and Previous Versions for that volume. They do not delete live files. Use /All only when shadows are stuck, or after an Event 25 purge when you want a clean slate.
vssadmin delete shadows /For=D: /Oldest vssadmin delete shadows /For=D: /All vssadmin delete shadows /All
6. Restart VSS after a failed snapshot
If list writers shows a failed or waiting writer after a purge:
net stop vss net stop swprv net start swprv net start vss vssadmin list writers
If a writer is still not Stable, reboot once before the next scheduled backup.
Volume Shadow Copy startup type can stay Manual. Microsoft Backup and third-party backup (including DATASTOR Shield) start it on demand.
7. GUI path (same settings)
- File Explorer → right-click the drive → Properties → Shadow Copies.
- Select the volume → Settings.
- Set “Located on this volume”, a maximum size, or “No limit”.
The schedule on that tab is Previous Versions of Shared Folders. It shares the same storage pool as backup snapshots.
8. Event log quick reference
| ID | Source | Meaning |
|---|---|---|
| 25 | Volsnap | All shadows on the volume deleted — storage could not grow in time (I/O or free space). |
| 33 | Volsnap | Oldest shadow deleted to stay under the user-defined max. |
| 36 | Volsnap | Shadows aborted because storage could not grow due to a user-imposed limit (max too small). |
| — | Backup log | 0x80042308 (VSS_E_OBJECT_NOT_FOUND) — snapshot object gone; usually follows Event 25. |
9. Checklist before the next backup
- Volume being snapshotted has at least 10–15% free space (not just a large MaxSize).
vssadmin list writers— every writer Stable / No error.vssadmin list shadowstorage— Used is not sitting against Maximum onC:or the data volume.- No large copy, delete, defrag, or full antivirus scan scheduled in the backup window.
- If Previous Versions is enabled on the data volume, reduce the schedule or cap so it cannot consume the whole pool.
- Re-run a test backup or wait for the next scheduled job and confirm the snapshot is created.
10. Worked example (typical 2 TB data volume)
Layout: Volume D: is about 2.15 TB. Shadow storage lives on D:. Maximum is 215 GB (10%). Used is small after Windows deleted the copies. C: is at 7.5 GB used of an 8.6 GB max.
Safe next steps:
vssadmin list writers vssadmin resize shadowstorage /For=D: /On=D: /MaxSize=50GB vssadmin resize shadowstorage /For=C: /On=C: /MaxSize=15GB vssadmin list shadowstorage
Keep several hundred GB free on D:. A 50 GB cap limits Previous Versions; it does not replace free space. If another local volume with spare capacity exists, relocate D: storage there instead of only shrinking the cap.
DATASTOR note
Backup software depends on Microsoft VSS for open files. Shadow storage size, free space, and I/O load are Windows settings. DATASTOR support can advise on these commands; changing them is performed by the customer’s administrator on the protected machine.