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VSS_E_ASRERROR_CRITICAL_DISKS_TOO_SMALL
Not enough suitably sized disks exist for all critical disks. Automated System Recovery cannot find a complete target set in which the number of available disks is at least the number of critical source disks and every assigned target is equal to or larger than its source disk.
The documented condition is: “Failed to find enough suitable disks for recreating all critical disks. The number of available disks should be same or greater than the number of critical disks at time of backup, and each one of the disks must be of same or greater size.”
Where VSS_E_ASRERROR_CRITICAL_DISKS_TOO_SMALL appears
The error is produced by global critical-disk feasibility checking. It is not enough for aggregate target capacity to exceed aggregate source capacity; ASR must be able to map each critical disk to an individual suitable target.
Likely causes
- The recovery computer exposes fewer eligible disks than the backup recorded as critical.
- At least one large critical source disk has no equal-or-larger candidate.
- Several small target disks provide enough total capacity but cannot replace one larger source disk.
- Offline, excluded, shared or removable disks reduce the eligible set below the required matching.
Evidence to capture before changing storage
- Build a table of every critical source disk and every eligible target with exact sizes.
- Record which disks were excluded and why they were considered ineligible.
- Capture controller and recovery-environment inventory after all intended hardware is attached.
- Save the attempted matching so the unmatched source disk is visible.
Diagnostic sequence
- Sort source disks and targets by size and test whether each source has a distinct adequate target.
- Do not combine capacities across target disks unless the documented recovery design supports that topology.
- Bring verified offline candidates online and rescan.
- Check whether a mistakenly selected critical volume inflated the required disk set.
Recovery and retry
Add larger targets, add missing targets, or correct the critical-volume selection. A retry is justified only after the matching inputs change. Repartitioning a target cannot increase its physical capacity.
What this HRESULT does not prove
The code does not identify a single failing disk and does not imply that backup files consume all of the source capacity. The comparison is based on recoverable disk topology.
Difference from nearby VSS results
VSS_E_ASRERROR_DISK_ASSIGNMENT_FAILED is the assignment outcome; this value emphasizes the count and per-disk size constraints that make a complete assignment impossible.
Practical example
Three critical source disks require 256 GB, 1 TB and 2 TB. Targets of 512 GB, 1 TB and 1.5 TB have more than 3 TB in total, but none can receive the 2 TB source, so ASR rejects the set.
Guidance for backup software and telemetry
Represent the problem as bipartite disk matching rather than aggregate free-space arithmetic. That model leads directly to useful logs and prevents misleading remediation.
Official Microsoft references
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