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

This drive failure risk estimator calculates the probability of losing data from your NAS array based on drive count, age, 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-8 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

4
AFR = % of drives of this type that fail per year. Source: manufacturer specs, cross-referenced with Backblaze drive stats.
3 years
Calculator uses uniform AFR across the period, a good approximation for years 1–4.
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…
Failure Probability
—
at least one drive over period
Expected Failures
—
drives over period
RAID Protection
—
failures tolerated

Core formula: P(at least one failure) = 1 - (1 - AFR)^(drives × years)

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

Rebuild window risk (for 1-tolerance RAID): 1 - (1 - AFR/8760 × rebuild_hours)^(drives - 1). This is the probability a second drive fails while the first is being rebuilt.

Worked example: 6 drives, 0.73% AFR, 3 years, 48hr rebuild. P(at least one) = 1 - 0.9927^18 = 12.4%. Expected = 6 × 3 × 0.0073 = 0.13 drives. Rebuild risk = 1 - (1 - 0.0073/8760 × 48)^5 = 0.020%.

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 March 2026.

AU Reference: AFR Data and Data Recovery Costs

Drive AFR Reference Table

Drive CategoryTypical AFRExample ModelsSource
Consumer HDD1.0-4.2%WD Blue, Seagate BarracudaManufacturer specs, Backblaze 2024
NAS HDD (standard)0.5-1.0%WD Red Plus, Seagate IronWolfManufacturer specs
NAS HDD (pro/CMR 7200rpm)0.35-0.5%WD Red Pro, Seagate IronWolf ProManufacturer specs
Enterprise HDD0.25-0.44%WD Gold, Seagate Exos X18Manufacturer specs
NAS SATA SSD0.1-0.35%Seagate IronWolf 110, WD Red SA500Manufacturer specs
Consumer SATA SSD0.1-0.5%Samsung 870 EVOManufacturer specs, Backblaze

Backblaze's annual drive stats reports (backblaze.com/cloud-storage/resources/hard-drive-test-data) provide real-world AFR data across millions of drives, a valuable cross-reference for manufacturer specs. Verified March 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 $500–$2,500+ depending on array size and failure type.

ProviderTypeApprox CostNotes
Payam Data RecoverySpecialist, AU-based$500–$2,500+Longest-established AU NAS recovery specialist
Ontrack (AU)Global specialist, AU office$1,000–$3,500+Higher cost, complex RAID cases
Secure Data RecoveryMid-tier AU service$400–$2,000Multiple 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 0.73% AFR, this probability is approximately 0.02% — 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 typically costs $500–$2,500 for standard RAID failures, and $1,000–$3,500+ for complex 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 millions 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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