Drive Failure Risk Estimator: How Likely Is a NAS Drive Failure?

This estimator answers three separate questions about your NAS array: how likely it is that at least one drive fails over the next 1 to 5 years, how many failed drives to expect, and, if one does fail, how risky the time is until the array is whole again.

A drive failure in a RAID array is a repair job, not data loss. Data is lost when more drives fail than the array can survive before the first one is replaced and rebuilt. So this page keeps the yearly odds of a failure apart from the risk during the degraded window that follows one, and it uses failure rates with a named source and a checked date.

Not sure how long your rebuild takes? The RAID Rebuild Time Estimator works it out and sends it here. For how the RAID levels themselves work, see RAID explained.

Your array

Number of drives

RAID type

Drive failure rate (AFR)

Custom AFR (% a year)

Time period

3 years

Uses one constant failure rate across the period. Real rates vary by model, batch and age.

If a drive fails

Wait for the replacement

Wait (hours)

Rebuild time (hours)

From your NAS's own progress display if you have seen one rebuild, or from the RAID Rebuild Time Estimator.

Results

Rebuild-window risk

Calculating…

Chance at least one drive fails

…

Expected drive failures

…

replacements to plan for, with failed drives replaced

Failures it survives at once

…

One more failure in the window

…

leaves the array with less redundancy, data intact

Assumes drives fail independently at a constant rate, so the rebuild-window figure is a simplified planning estimate, not a forecast. Real risk can differ a lot, and some of what it leaves out can make it higher: drives from the same batch, shared heat and the rebuild's own load make real failures cluster, and read errors are not counted.

Estimate this array's rebuild time →

How we calculate

Every figure assumes each drive fails independently at a constant yearly rate, the AFR you pick. That is a planning model: real failure rates differ by model, production batch, temperature and age, and they do not follow one universal curve.

Chance at least one drive fails in y years with n drives: 1 − (1 − AFR)^{n × y}.
Expected drive failures: n × y × AFR. It counts replacements, assuming each failed drive is swapped for one with the same rate.
Chance one drive fails within a window of h hours: p = 1 − (1 − AFR)^{h ÷ 8,760}.
Rebuild-window risk: once one drive has failed, the window is the wait for the replacement plus the rebuild. With m drives left, RAID 5, SHR, RAIDZ1 and Unraid single parity lose data if any one of them fails (1 − (1 − p)^m). RAID 6, SHR-2, RAIDZ2 and Unraid dual parity need two more failures, RAIDZ3 three (a binomial sum). RAID 10 loses data only if the failed drive's own mirror partner fails, about p. A mirror of three or more drives needs every remaining copy to fail (p^m).
Bands: under 0.1% is Low, 0.1% to under 1% is Moderate, 1% and over is High. The bands are set low on purpose: the figure is a simplified planning estimate, and some of what it leaves out, such as failures that cluster and read errors, can make the real risk higher.
MTBF, where a vendor publishes only that, becomes an implied rate with AFR = 1 − e^{−8,760 ÷ MTBF} and is labelled derived.

What it does not calculate: a multi-year chance of losing data. Two failures months apart, with a finished rebuild between them, are two repairs, so counting failures over years overstates the risk many times over. Unreadable sectors met during a rebuild are a separate question, covered by the Rebuild Time Estimator.

Worked example

Six drives in RAID 5 at 0.87% AFR, over 3 years, with a 48 hour degraded window (for example a hot spare and a 48 hour rebuild).

Chance at least one drive fails in 3 years: 1 − 0.9913¹⁸ = 14.6%.
Expected failures: 6 × 3 × 0.0087 = 0.16 drives.
If one fails, the chance one of the other five fails in the 48 hours: 0.024%, about 1 in 4,200. Band: Low.
The same six drives in RAID 6 need two of the five to fail in those 48 hours: under 0.0001%, less than 1 in 10,000,000. The price is one more drive of capacity.

The first figure says you should expect to replace a drive at some point. The window figure says how exposed you are while you do it. Neither one is the chance of losing your data over three years.

Drive reliability evidence

The rate presets above come from this table. Each figure was read from the vendor's own datasheet or manual, or from Backblaze's published fleet statistics, and the quote was re-checked on the source page. Where a vendor rates different capacities of one drive line differently, each capacity band has its own row and its own preset, so pick the band your drives are in. A figure we could not confirm at its source is left out, not estimated.

Drive or dataset	Evidence type	Value	What it means	Source checked
Seagate IronWolf	Vendor-published AFR	0.87% a year	The vendor's own annualised failure rate, under the operating conditions its document states.	Source, 2026-10-06
Seagate IronWolf	Vendor MTBF	1,000,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.87% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-06
Seagate IronWolf	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
Seagate IronWolf	Rated workload	180 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-06
Seagate IronWolf	Vendor limited warranty	3 years	The manufacturer's own warranty term.	Source, 2026-10-06
Seagate IronWolf Pro	Vendor-published AFR	0.35% a year	The vendor's own annualised failure rate, under the operating conditions its document states.	Source, 2026-10-06
Seagate IronWolf Pro	Vendor MTBF	2,500,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.35% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-06
Seagate IronWolf Pro	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
Seagate IronWolf Pro	Rated workload	550 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-06
Seagate IronWolf Pro	Vendor limited warranty	5 years	The manufacturer's own warranty term.	Source, 2026-10-06
Seagate Exos (Exos M)	Vendor-published AFR	0.35% a year	The vendor's own annualised failure rate, under the operating conditions its document states.	Source, 2026-10-06
Seagate Exos (Exos M)	Vendor MTBF	2,500,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.35% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-06
Seagate Exos (Exos M)	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
Seagate Exos (Exos M)	Rated workload	550 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-06
Seagate Exos (Exos M)	Vendor limited warranty	5 years	The manufacturer's own warranty term.	Source, 2026-10-06
Seagate Barracuda	Read-error rating (URE)	1 per 10¹⁴ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
Seagate Barracuda	Rated workload	120 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-06
Seagate Barracuda	Vendor limited warranty	2 years	The manufacturer's own warranty term.	Source, 2026-10-06
WD Red Plus	Vendor MTBF	1,000,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.87% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-06
WD Red Plus	Read-error rating (URE)	1 per 10¹⁴ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
WD Red Plus	Rated workload	180 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-06
WD Red Plus	Vendor limited warranty	3 years	The manufacturer's own warranty term.	Source, 2026-10-06
WD Red Pro 2 to 12 TB	Vendor MTBF	2,000,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.44% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-12
WD Red Pro 2 to 12 TB	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
WD Red Pro 2 to 12 TB	Rated workload	550 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-12
WD Red Pro 2 to 12 TB	Vendor limited warranty	5 years	The manufacturer's own warranty term.	Source, 2026-10-12
WD Red Pro 14 to 26 TB	Vendor MTBF	2,500,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.35% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-12
WD Red Pro 14 to 26 TB	Rated workload	550 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-12
WD Red Pro 14 to 26 TB	Vendor limited warranty	5 years	The manufacturer's own warranty term.	Source, 2026-10-12
WD Red Pro 14 to 26 TB except WD141KFGX	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
WD Red Pro WD141KFGX (14 TB)	Read-error rating (URE)	1 per 10¹⁴ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
WD Blue (3.5" desktop)	Read-error rating (URE)	1 per 10¹⁴ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-06
WD Blue (3.5" desktop)	Vendor limited warranty	2 years	The manufacturer's own warranty term.	Source, 2026-10-06
Toshiba N300 4 to 10 TB	Vendor MTTF	1,000,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.87% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-12
Toshiba N300 4 to 10 TB	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
Toshiba N300 4 to 10 TB	Rated workload	180 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-12
Toshiba N300 4 to 10 TB	Vendor limited warranty	3 years	The manufacturer's own warranty term.	Source, 2026-10-12
Toshiba N300 12 to 18 TB	Vendor MTTF	1,200,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.73% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-12
Toshiba N300 12 to 18 TB	Read-error rating (URE)	1 per 10¹⁴ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
Toshiba N300 12 to 18 TB	Rated workload	180 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-12
Toshiba N300 12 to 18 TB	Vendor limited warranty	3 years	The manufacturer's own warranty term.	Source, 2026-10-12
Toshiba N300 20 to 22 TB	Vendor MTTF	1,200,000 hours	A population statistic, not a prediction for one drive. Converted, it implies about 0.73% a year: a figure NTK derived, not one the vendor published.	Source, 2026-10-12
Toshiba N300 20 to 22 TB	Read-error rating (URE)	1 per 10¹⁵ bits read	A worst-case specification for unreadable sectors, not a measured error rate.	Source, 2026-10-12
Toshiba N300 20 to 22 TB	Rated workload	180 TB a year	The data volume a year the vendor rates the drive for.	Source, 2026-10-12
Toshiba N300 20 to 22 TB	Vendor limited warranty	3 years	The manufacturer's own warranty term.	Source, 2026-10-12
Backblaze Drive Stats, Q2 2026	Observed fleet AFR	1.73%	All drives in Backblaze fleet analysed, quarter (Apr 1-Jun 30 2026). A data-centre fleet: context, not a rating for a home NAS drive.	Source, 2026-10-06
Backblaze lifetime fleet AFR, to June 30 2026	Observed fleet AFR	1.41%	All drives in Backblaze fleet, lifetime to 2026-06-30. A data-centre fleet: context, not a rating for a home NAS drive.	Source, 2026-10-06
Backblaze Drive Stats, 2025 annual	Observed fleet AFR	1.36%	All drives in Backblaze fleet, calendar year 2025. A data-centre fleet: context, not a rating for a home NAS drive.	Source, 2026-10-06

Vendor AFR, MTBF and MTTF figures assume the vendor's stated conditions (for example power-on hours, temperature and workload). Observed fleet rates come from data-centre drives and are context, not a rating for a home NAS drive. A vendor warranty term is the manufacturer's own promise. Your first step for a faulty product is usually the retailer that sold it. The ACCC explains your rights under the Australian Consumer Law at accc.gov.au.

Single or dual parity: the trade-off

There is no drive size at which RAID 5 stops being reasonable for everyone. What changes the answer is how long you would run degraded and what a loss would cost you. Dual parity (RAID 6, SHR-2, RAIDZ2) is worth more as these go up:

Rebuild time: bigger drives and busier arrays rebuild for longer, so the window grows.
Drive count: more remaining drives means more that can fail during the window.
Same-age drives: drives bought together tend to age together.
Replacement wait: days spent ordering a drive count as degraded time too.
Backup quality: with a current, tested backup, a loss costs restore time. Without one, it costs the data.

The cost of dual parity is one more drive of capacity. Put your own numbers into the calculator: if the single-parity band reads Moderate or High, the second parity drive is doing real work.

Hot spare, cold spare or dual parity

They shrink different parts of the risk. A hot spare sits in the NAS and starts the rebuild straight away, removing the wait for a replacement, but adds no redundancy and uses a bay and power. A cold spare on the shelf removes the shipping wait at the cost of a trip to the NAS. Dual parity lets the array survive a second failure during the window itself. Change the wait setting above to see what each removes.

Drive health warnings

Run the health checks your NAS offers (SMART tests, scrubs) and replace a drive that reports growing reallocated or pending sectors. Some failures are preceded by warnings like these and many are not, so a clean result is not a guarantee the drive will survive a rebuild.

RAID is not a backup. RAID keeps an array running through drive failures. It does not protect against deletion, ransomware, a failed rebuild, theft, fire or flood. See our 3-2-1 Backup Strategy Guide. If a rebuild does fail and there is no backup, professional recovery is possible in many cases: Australian providers include Payam Data Recovery, Ontrack and Secure Data Recovery, and quotes vary by failure type. Our guide to NAS data recovery costs in Australia covers what drives the price.

Frequently asked questions

What is AFR and how does it turn into a failure chance?

AFR (annualised failure rate) is the share of drives of a type expected to fail in a year. At a constant rate, the chance that at least one of n drives fails within y years is 1 minus (1 minus AFR) to the power of n times y. Four drives at 1% AFR over 3 years gives 11.4%. That is the chance you will be replacing at least one drive, not the chance of losing data.

Why doesn't this calculator show a chance of losing data over several years?

Because counting failures over years does not tell you whether data is lost. If one drive in a RAID 5 array fails in March and is replaced and rebuilt, a second failure in October is a second repair, not data loss. Data is lost when further failures land while the array is still degraded. So the calculator shows the yearly odds of a failure, and separately the risk during the degraded window that follows one.

What is the rebuild-window risk?

After a drive fails, the array runs degraded until the replacement is in and the rebuild finishes. The rebuild-window risk is the chance, given that first failure, that enough of the remaining needed drives also fail in that window to lose data. For RAID 5 that is any one more drive; for RAID 6 it is two more; for RAID 10 it is the failed drive's mirror partner. A shorter wait for the replacement and a faster rebuild both shrink it.

Is MTBF the same as AFR?

No. MTBF (mean time between failures) is a population statistic that vendors publish for some drives, and vendors say it does not predict how long one drive will last. It can be converted to an implied yearly rate with AFR = 1 minus e to the power of (minus 8,760 divided by MTBF), so 1,000,000 hours implies about 0.87% a year. Some vendors publish MTTF (mean time to failure) instead, and the calculator converts it the same way. Where it uses a converted figure, it names the vendor's term and says the rate was derived, not published as an AFR.

Does Backblaze data tell me how reliable my NAS drive is?

Only as context. Backblaze publishes observed failure rates for the drives in its own data centres, which are mostly large enterprise models running around the clock. Where your exact model is in their table with a large sample, its observed rate is useful evidence. The fleet-wide figure is not a rating for a home NAS drive, and it does not confirm or replace a vendor's figure for a different model.

Does RAID protect against all data loss?

No. RAID keeps an array running through the drive failures it is built to survive. It does not protect against deleting the wrong folder, ransomware, a failed rebuild, theft, fire or flood. Keep a separate backup, ideally following the 3-2-1 rule: three copies, on two kinds of storage, with one off site. See our 3-2-1 Backup Strategy Guide.

What happens if a rebuild fails and I need data recovery?

Restore from your backup first. If there is no usable backup, professional NAS and RAID recovery is possible in many cases. Australian providers include Payam Data Recovery, Ontrack and Secure Data Recovery, and quotes vary by failure type and drive count. Our guide to NAS data recovery costs in Australia covers what drives the price.