What does HRESULT 0xC0380055 (ERROR_VOLMGR_NUMBER_OF_DISKS_IN_MEMBER_INVALID) mean?

ERROR_VOLMGR_NUMBER_OF_DISKS_IN_MEMBER_INVALID

What this result says about the disk model: volmgr number of disks in member invalid

ERROR_VOLMGR_NUMBER_OF_DISKS_IN_MEMBER_INVALID (0xC0380055) is emitted by the volmgr path when the member descriptor names an unsupported number of contributing disks or extents. The deciding object for this HRESULT is dynamic-volume plex member descriptor and its backing extents, so the first investigation belongs there rather than in a generic “disk failure” bucket for this HRESULT.

Dynamic volumes are represented as packs containing disks and volumes; each volume has one or more plexes, and each plex is assembled from members and extents. Indices and counts are relative to the submitted layout generation, while configuration logs are replicated across participating dynamic disks. This object model determines which identifiers, counts and ownership state are meaningful.

The standard message identifies the immediate condition, but this result also requires the producer API, object topology and attempted state transition for this HRESULT. Capture them before an automatic refresh advances the storage generation.

Capture the exact object generation

Do not overwrite this result with a close, detach or rollback error. Preserve the causal order so the original storage contract remains visible.

Prove the boundary in a test volume

1. Create a nonproduction case for this HRESULT that preserves the same geometry and ownership model.
2. Collect pack ID, volume ID, plex index, member index, disk IDs, extent offsets, health state and regeneration percentage before invoking the first mutating API.
3. Change one condition only: serialize the exact VM_VOLUME_LAYOUT buffer, validate every count and Size field, and compare it with a layout produced by a known-good management API for this HRESULT.
4. Re-enumerate after the transition and confirm this result is not reproduced by a stale handle from the old generation.

Use ERROR_VOLMGR_UNEXPECTED_DISK_LAYOUT_CHANGE as the nearest comparison. This result is specific to the member descriptor names an unsupported number of contributing disks or extents; the neighboring result belongs to another validation or lifecycle branch. Keeping both names prevents a generic storage label from hiding the actual owner.

Mechanism-specific note

The count field is part of the layout contract rather than an advisory value. Verify it against the number of serialized descriptors and against the minimum geometry required by the selected layout type while diagnosing it. Check multiplication and addition for overflow before allocating or copying the variable-length buffer, because a correctly sized allocation can still contain a logically impossible count.

Safe recovery path

Rebuild the layout from enumerated objects and checked arithmetic rather than patching one serialized field; validate counts, indices, sizes, alignment and overflow before submission. The result repair is incomplete until a fresh enumeration and new handle observe the corrected state for this HRESULT.

• The controlled A/B case changes only the invariant responsible for this HRESULT.
• No unrelated disk, cluster owner, tier policy or virtual-disk backing file is altered to hide it.
• Health, synchronization or job state reaches a terminal value consistent with the repaired the operation.
• Telemetry includes both symbolic names when it or ERROR_VOLMGR_UNEXPECTED_DISK_LAYOUT_CHANGE occurs again.

A reboot, blanket disk conversion or deletion of storage metadata is not a justified first response to it; it changes multiple invariants and can erase the evidence needed to identify the defect while diagnosing it.

Technical references

• Microsoft: COM error codes for VOLMGR, VHD and SDIAG — defines the volmgr objects used to interpret it.
• Microsoft: Virtual Disk Service object model — primary API or object-model reference for the boundary.
• Microsoft: Pack object — use this source to verify volmgr state and parameters for this HRESULT.
• Microsoft: Virtual Disk Service common return codes — documents the supported management path relevant to it.

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