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 is dynamic-volume plex member descriptor and its backing extents, so the first investigation belongs there rather than in a generic “disk failure” bucket.

Capture the exact object instance

Prove the condition in a test volume

1. Create a nonproduction case 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.
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. 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 repair is incomplete until a fresh enumeration and new handle observe the corrected state.

• The controlled A/B case changes only the invariant that triggers this result.
• 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 operation.
• Telemetry includes both symbolic names when it or ERROR_VOLMGR_UNEXPECTED_DISK_LAYOUT_CHANGE occurs again.

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.
• Microsoft: Pack object — use this source to verify volmgr state and parameters.
• Microsoft: Virtual Disk Service common return codes — documents the supported management path relevant to this HRESULT.