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.