This Unraid parity rebuild and parity check estimator calculates how long a parity sync or disk rebuild will take based on array size and drive speed. Also shows the difference between correcting and non-correcting parity check modes.
When a disk fails in Unraid, the array rebuilds the missing disk's contents by reading every other disk and using parity to reconstruct the data. Until that rebuild finishes, you're running degraded, with single parity, a second disk failure during the rebuild can be catastrophic; with dual parity, you still have one remaining layer of protection, but the risk window is real either way. This tool estimates your rebuild duration, explains the risk you're carrying, and tells you what to do about it.
Operation Type
Rebuilding a failed data disk. Parity + all remaining disks are read to reconstruct the missing data. The time estimate depends on disk size and read speed.
Drive + Array
Array Conditions
SMR drives and Unraid: a known problem. SMR (Shingled Magnetic Recording) drives write new data over old tracks, causing severe slowdowns under sustained write load, exactly what a parity rebuild requires. Rebuild times can be 2–3× longer than a CMR drive of the same size, and some SMR drives have been known to drop offline mid-rebuild. Unraid and most NAS vendors officially recommend CMR drives for parity and data disks. Community discussion on rebuild speeds.
Estimated Rebuild Duration
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Estimated rebuild window
Actual speed is usually limited by the slowest participating disk and your controller or HBA.
During a rebuild, Unraid is reading every sector of every remaining drive. That sustained read load is the most stress your drives will see outside of a manufacturer test. If a second drive has a sector it can't read, the rebuild fails, and without dual parity, that means data loss.
Drive Size
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Drive Type
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Array Size
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Est. Read Speed
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After activity penalty
Risk-Reduction Actions
Schedule the rebuild when you can monitor it. Don't kick it off before you leave for a week.
Reduce array activity. The rebuild will finish faster and your drives will thank you.
Check SMART status on all remaining drives before starting. Look for reallocated sectors or pending sectors.
Consider an interim backup. If the data matters, a cold copy to an external drive before you pull the failed disk is cheap insurance.
Array + Drive
Unraid's default is correcting mode. For routine scheduled checks on a healthy array, consider switching to read-only to audit without automatically overwriting mismatches. Run correcting mode manually after an unclean shutdown or once errors are confirmed.
Workload
Estimated Parity Check Duration
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Estimated check window
Scheduling Recommendation
Performance Impact
If your parity check finds errors: Finding errors after an unclean shutdown (power cut, hard reset, crash) is normal. A correcting check fixes this. Finding errors after a clean shutdown is worth investigating, possible causes include a failing drive or controller issue.
Methodology: Rebuild and parity check durations are estimated based on the sequential read speed of your drives and the size of the largest drive in the array. SMR speed penalties are based on community-reported rebuild times from the Unraid forums and manufacturer sustained-write specs. Activity penalties are approximations based on observed I/O contention. We show ranges rather than single numbers because the margin of error in any single estimate is wide. Last reviewed: 20 March 2026.
Frequently Asked Questions
Why does my rebuild take so much longer than the estimate?
Most common reasons: SMR drive slowing sustained reads, saturated HBA or SATA controller, heavy array activity. Check per-drive read speeds in Unraid's dashboard, if one drive is significantly slower, that's your culprit.
Does running a parity check damage my drives?
Not directly, sustained reads are within normal operating parameters. But parity checks reveal pre-existing issues. This is why parity checks are valuable, not something to avoid.
How often should I run a parity check?
Unraid defaults to monthly. For most home users that's reasonable. After any unclean shutdown, run a correcting check.
What's the difference between a parity check with corrections and without?
A correcting check rewrites parity to fix mismatches. A read-only check reports them without changing anything. For routine scheduled checks on a healthy array, non-correcting (read-only) is recommended, review the results first, then run correcting manually if errors are found. Always run correcting after an unclean shutdown.
My parity check is showing errors. What does that mean?
Usually parity is out of sync from an unclean shutdown. A correcting check fixes this. Errors after a clean shutdown warrant further investigation, check SMART data and consider a RAM test.
AU NAS Drive Pricing Reference: Parity & Data Drives (early 2026)
During a parity rebuild, every sector of every remaining drive is read continuously for hours. Choosing NAS-rated drives rated for NAS workloads reduces the risk of a second failure mid-rebuild. AU retail prices from Mwave, PLE, Scorptec.
Drive
Capacity
AU retail range
Retailer
Seagate IronWolf
4 TB
$135-$165
Mwave, PLE, Amazon AU
Seagate IronWolf
8 TB
$235-$280
Mwave, PLE
Seagate IronWolf
12 TB
$330-$380
Mwave, PLE, Amazon AU
WD Red Plus
4 TB
$130-$165
Mwave, PLE, Amazon AU
WD Red Plus
8 TB
$225-$270
Mwave, PLE
WD Red Pro
8 TB
$300-$350
Mwave, Scorptec
WD Red Pro
16 TB
$520-$600
Mwave, PLE
AU Cold Spare Strategy
A cold spare is an identical (or compatible) drive kept in reserve so that when a parity rebuild completes, you can immediately replace the failed drive rather than waiting for AU retail shipping. Standard shipping from Mwave, PLE, or Scorptec is typically 2-5 business days, extending your window of degraded protection.
Drive cost for a cold spare is the same as the table above. For a 4-bay array with 8 TB drives, budget approximately $250-$280 AUD for a WD Red Plus spare.
ACL warranty note: NAS-rated drives from Australian retailers carry a 3-year warranty under the Australian Consumer Law, serviced locally. If a drive fails mid-rebuild, you are in a degraded state while waiting for a warranty return, keeping a cold spare prevents this gap entirely.