Drive Failure Risk Estimator — What Are the Odds Your NAS Drive Will Fail?

Q: What is the rebuild window risk and why does it matter?

During a RAID 5 rebuild, the array is degraded, it cannot tolerate another failure. The rebuild window risk is the probability that a second drive fails before the rebuild completes. For a 48-hour rebuild with 5 remaining drives at 0.87% AFR, this probability is approximately 0.024%, small but non-zero. This is why RAID 6 is recommended for 6+ drive arrays and for older drives.

This drive failure risk estimator calculates the probability of losing data from your NAS array based on drive count, time period, RAID level, and manufacturer AFR data. Shows annualised failure risk and how RAID level affects your data safety margin.

Calculate the real probability of a drive failure in your NAS array over 1, 2, or 3 years — and find out whether your RAID configuration gives you adequate protection.

Most NAS users either ignore failure risk entirely or assume RAID makes them safe. Neither is correct. Manufacturer-published AFR data gives us a real probability estimate — and the math shows that a 4-drive array at typical NAS HDD rates has roughly a 1-in-10 chance of at least one failure over 3 years. See our RAID Explained guide for a full overview of how RAID protection works.

Your Array

Number of Drives

Drive AFR

Custom AFR (%)

AFR = % of drives of this type that fail per year. Source: manufacturer specs, cross-referenced with Backblaze drive stats.

Array Age / Time Period

3 years

Calculator uses uniform AFR across the period, a good approximation for years 1–4.

RAID Configuration

Estimated Rebuild Time

Use our RAID Rebuild Time Estimator to estimate your actual rebuild time. During a rebuild, a second failure = data loss on RAID 5/SHR.

Risk Assessment

Calculating…

Data Loss Probability

—

at least one drive over period

Expected Failures

—

drives over period (assumes failed drives are replaced and the replacement carries the same failure rate)

RAID Protection

—

failures tolerated

Data-loss probability is RAID-aware. The headline number is not just "at least one drive fails" — it's the probability of enough simultaneous, unrepaired failures to actually lose data, given how many failures your RAID level tolerates:

No redundancy (JBOD): P(≥1 failure) = 1 - (1 - AFR)^(drives × years)
Tolerates 1 failure (RAID 1/5/SHR/single-parity): P(≥2 failures), via the binomial distribution
RAID 10: not the same binomial model — data is only lost if both drives in the same mirror pair fail, so it uses P(any mirror pair loses both members), which is meaningfully lower than the generic P(≥2 failures) figure at the same drive count
Tolerates 2 failures (RAID 6/SHR-2/dual-parity): P(≥3 failures), via the binomial distribution

This assumes independent failure events and constant AFR across the period. It's an approximation that slightly understates risk in year 1 (infant mortality) and years 5+ (wear-out), and is accurate for years 2–4.

Expected failures: drives × years × AFR — assumes any failed drive is replaced and the replacement carries the same failure rate.

Rebuild window risk: 1 - (1 - AFR)^((rebuild_hours / 8760) × exposed_drives), where exposed_drives is the number of drives whose failure during the rebuild would cause data loss — every other member for RAID 5/SHR/single-parity (drives - 1), but only the failed drive's mirror partner for RAID 10 (1), since a failure in a different mirror pair doesn't cause data loss.

Worked example: 6 drives, RAID 5, 0.87% AFR (Standard NAS HDD preset), 3 years, 48hr rebuild. P(≥1 failure) = 1 - 0.9913^18 ≈ 14.6%, but RAID 5 tolerates one failure, so the actual data-loss probability is P(≥2 failures) ≈ 1.06%. Expected = 6 × 3 × 0.0087 ≈ 0.16 drives. Rebuild risk = 1 - (1 - 0.0087)^((48/8760) × 5) ≈ 0.024%.

Configurations below a RAID level's minimum drive count (e.g. single-drive SHR, 2-drive RAID 6) aren't valid and are flagged rather than calculated.

Source: AFR figures from manufacturer published specifications (WD, Seagate), cross-referenced with Backblaze annual drive stats reports (backblaze.com/cloud-storage/resources/hard-drive-test-data). Verified August 2026.

AU Reference: AFR Data and Data Recovery Costs

Drive AFR Reference Table

Drive Category	Typical AFR	Example Models	Source
Consumer HDD	1.0-4.2%	WD Blue, Seagate Barracuda	Manufacturer specs, Backblaze 2024
NAS HDD (standard)	0.5-1.0%	WD Red Plus, Seagate IronWolf	Manufacturer specs
NAS HDD (pro/CMR 7200rpm)	0.35-0.5%	WD Red Pro, Seagate IronWolf Pro	Manufacturer specs
Enterprise HDD	0.25-0.44%	WD Gold, Seagate Exos X18	Manufacturer specs
NAS SATA SSD	0.1-0.44%	Seagate IronWolf 110, WD Red SA500	Manufacturer specs
Consumer SATA SSD	0.1-0.5%	Samsung 870 EVO	Manufacturer specs, Backblaze

Backblaze's annual drive stats reports (backblaze.com/cloud-storage/resources/hard-drive-test-data) provide real-world AFR data across hundreds of thousands of drives, a valuable cross-reference for manufacturer specs. Verified August 2026.

AU Data Recovery Providers

If a second drive fails during a RAID 5 rebuild, professional data recovery may be possible. In Australia, NAS data recovery costs range from around $800 for a simple case to $10,000+ for complex, multi-drive, or physically damaged arrays — pricing is usually tiered by complexity, not a flat rate.

Provider	Type	Approx Cost	Notes
Payam Data Recovery	Specialist, AU-based	$800–$10,000+	Tiered: ~$800–$2,500 simple, $2,500–$4,500 common, $4,500–$10,000 complex, $10,000+ rare/emergency
Ontrack (AU)	Global specialist, AU office	$1,000–$3,500+	Higher cost, complex RAID cases
Secure Data Recovery	Mid-tier AU service	$400–$2,000	Multiple AU cities

These costs make a compelling argument for RAID 6 or dual-parity on 6+ drive arrays, and for 3-2-1 backup regardless of RAID level.

RAID is not a backup. RAID protects against hardware failure, it does not protect against ransomware, accidental deletion, fire, or flood. The 3-2-1 backup rule remains essential regardless of RAID level. See our 3-2-1 Backup Strategy Guide. In Australia, CGNAT affects some cloud backup options, see our NBN Remote Access Reality Checker.

Frequently Asked Questions

AFR (Annualised Failure Rate) is the percentage of drives of a specific model that fail per year. A 1% AFR means roughly 1 in 100 drives fail annually. For a 4-drive array at 1% AFR over 3 years, the probability of at least one failure is approximately 11.4% — calculated as 1 - (1 - 0.01)^(4×3). This is an approximation assuming constant AFR across the period.

During a RAID 5 rebuild, the array is degraded — it cannot tolerate another failure. The rebuild window risk is the probability that a second drive fails before the rebuild completes. For a 48-hour rebuild with 5 remaining drives at 0.87% AFR, this probability is approximately 0.024% — small but non-zero. This is why RAID 6 is recommended for 6+ drive arrays and for older drives. Use our RAID Rebuild Time Estimator to estimate your actual rebuild duration.

No. RAID protects against hardware failure of individual drives, but not against concurrent multi-drive failure, ransomware, accidental deletion, fire, or flood. The 3-2-1 backup rule remains essential regardless of RAID level: 3 copies of data, on 2 different media types, with 1 offsite copy. See our 3-2-1 Backup Strategy Guide.

Professional NAS data recovery in Australia is typically tiered by complexity: roughly $800–$2,500 for simple single-drive cases, $2,500–$4,500 for common multi-file problems, $4,500–$10,000 for complex RAID/multiple drive failures, and $10,000+ for rare or emergency cases. Named AU providers include Payam Data Recovery (longest-established AU NAS specialist), Ontrack (global with AU office), and Secure Data Recovery (multiple AU cities).

Backblaze publishes annual drive stats reports covering hundreds of thousands of drives in real-world use (backblaze.com/cloud-storage/resources/hard-drive-test-data). Manufacturer spec sheets also publish AFR figures. Backblaze data generally confirms manufacturer AFR figures for NAS-class drives, though conditions differ from home NAS use.

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