What does HRESULT 0xC05CFF0C (ERROR_VHDSET_BACKING_STORAGE_NOT_FOUND) mean?

ERROR_VHDSET_BACKING_STORAGE_NOT_FOUND

Why this is narrower than a generic disk error: vhdset backing storage not found

ERROR_VHDSET_BACKING_STORAGE_NOT_FOUND has unsigned value 0xC05CFF0C. In the virtdisk layer, the operation is rejected because a VHD Set references a backing storage file that cannot be resolved at its expected location or generation. Diagnosis of this result should follow VHD Set metadata and the backing storage file selected for the active disk through one controlled state transition.

The Virtual Disk API chooses a format provider from VIRTUAL_STORAGE_TYPE, opens an image with explicit access rights and can attach it as a disk. Provider selection, backing-chain integrity, host-file attributes, sector geometry and operating-system ownership are independent validation stages.

Storage facts that change the diagnosis

Keep this HRESULT as the primary result if rollback or cleanup reports another code; those later values describe recovery, not the first rejected invariant.

A/B test

1. Before changing state, freeze the failing generation and save image path, VIRTUAL_STORAGE_TYPE, provider/vendor IDs, VHD/VHDX/VHD Set identifiers, host sector sizes, attach flags, ownership and online/read-only state.
2. Exercise this exact comparison: enumerate the VHD Set references, restore one missing backing file at the recorded identity, and reopen the set without rewriting its metadata.
3. During the comparison, keep image bytes, unrelated disks, provider policy and cluster topology fixed.
4. After the test, record the next return value and the durable object state; crossing the boundary should expose a later result rather than silently retrying.

The distinction from ERROR_VIRTDISK_NOT_VIRTUAL_DISK is operational. A VHD Set references a backing storage file that cannot be resolved at its expected location or generation; log the neighboring code separately so capability, identity and state are not conflated.

Mechanism-specific note

VHD Set metadata selects one or more backing files by identity, not merely by a convenient filename. Enumerate every referenced member and search cluster storage using the recorded identity. Restoring an unrelated file under the missing name can satisfy path lookup but corrupt the relationship among VHD Set members.

Corrective action and regression check

Correct the provider, image format, backing-file chain, host geometry or disk ownership identified by the failing call, then reopen the image to obtain a new handle before retrying. After correcting it, reopen the pack, volume, tier, snapshot or virtual disk so no cached generation participates in the validation run.

• The original request now reaches success or a documented later status with the intended layout and access mode.
• A new enumeration reports internally consistent IDs, counts, geometry and ownership for the corrected object.
• The passing state survives the relevant lifecycle transition, such as reopen, remount, owner change or detach/attach.
• The application still distinguishes it from ERROR_VIRTDISK_NOT_VIRTUAL_DISK and has no unbounded retry loop.

Technical references

• Microsoft: Virtual Disk API — documents the supported management path relevant to this HRESULT.
• Microsoft: OpenVirtualDisk — defines the virtdisk objects used to interpret it.
• Microsoft: AttachVirtualDisk — primary API or object-model reference.
• Microsoft: GetVirtualDiskInformation — use this source to verify virtdisk state and parameters.