Quick troubleshooting view
Parity, mirroring, the smallest disk, and TB/TiB conversion commonly explain the gap.
- Identify the actual RAID level
- List exact disk sizes
- Calculate from the smallest disk
- Correct the capacity estimate using the actual RAID level and smallest disk
- Replace or regroup mismatched drives only after validating the storage design
- Document controller/filesystem overhead before resizing
Contextual technician plan
Perform this check and preserve the observed result before changing configuration.
The calculated usable capacity should match the actual RAID level, disk count, smallest-disk size, spare/reserve and TB/TiB conversion.
If the theoretical calculation matches the controller/OS values, the capacity is explained by the design rather than a fault.
If values still differ materially, preserve the controller layout and investigate hidden reserve, hot spare, mixed disk sizes or filesystem overhead before resizing.
Perform this check and preserve the observed result before changing configuration.
The calculated usable capacity should match the actual RAID level, disk count, smallest-disk size, spare/reserve and TB/TiB conversion.
If the theoretical calculation matches the controller/OS values, the capacity is explained by the design rather than a fault.
If values still differ materially, preserve the controller layout and investigate hidden reserve, hot spare, mixed disk sizes or filesystem overhead before resizing.
Perform this check and preserve the observed result before changing configuration.
The calculated usable capacity should match the actual RAID level, disk count, smallest-disk size, spare/reserve and TB/TiB conversion.
If the theoretical calculation matches the controller/OS values, the capacity is explained by the design rather than a fault.
If values still differ materially, preserve the controller layout and investigate hidden reserve, hot spare, mixed disk sizes or filesystem overhead before resizing.
Repeat the same validation test after the correction and confirm the original symptom is gone. Validate stability before closing the incident.
Before changing configuration, record the current value and a way back.
Escalate before changing an active RAID layout or when a degraded array is also reported.
＋Open the complete detailed guideDetailed explanations and original troubleshooting content.
Parity, mirroring, the smallest disk, and TB/TiB conversion commonly explain the gap.
Likely causes
- RAID 5/6 consumes parity capacity
- RAID 10 halves capacity
- Mixed disk sizes
- System reserve or hot spare
Checks in priority order
- Identify the actual RAID level
- List exact disk sizes
- Calculate from the smallest disk
- Distinguish decimal TB from binary TiB
When to escalate
Escalate when the failure affects multiple users, a production dependency is unavailable, or logs show a component outside your control. Include timestamps, scope, tests already performed, and the last known working state.