Why RAID Arrays Fail: 8 Most Common Scenarios
RAID was supposed to protect your data. It was supposed to survive disk failure. It was supposed to be reliable. And yet it failed.
There are many causes of RAID failure, and not all are related to disks. Studies have shown a surprising fact: most RAID failures are not caused by faulty hardware, but by human error.
In this article, we'll examine 8 most common scenarios and how to prevent them.
1. Multiple Disk Failure
What It Means
Failure of more disks than the RAID configuration tolerates:
- RAID 5: 2+ failed disks
- RAID 6: 3+ failed disks
- RAID 10: both disks in one mirror pair
Why It Happens
Disks from the same production batch: If you buy 8 disks at once, they're likely from the same batch. They have similar characteristics – including similar lifespan. If one fails after 4 years, the others are probably close.
Domino effect during rebuild: When one disk fails and you start rebuild, the remaining disks are under extreme load. Rebuild reads 100% of all sectors. For disks near end of life, this can be the last straw.
Ignored degraded status: Company ignores "RAID Degraded" warnings for months. Another disk fails and data is gone.
Prevention
- Buy disks from different batches
- Proactively replace old disks (4-5 years)
- Never ignore degraded status
- Consider RAID 6 instead of RAID 5 (tolerates 2 failures)
Recovery
With professional equipment, RAID with 2 failed disks can often be reconstructed. Success depends on extent of damage.
2. Controller Failure
What It Means
The RAID controller – hardware that manages the array – fails. Array disappears as if it never existed.
Causes
Electrical damage: Surge, short circuit, faulty power supply can burn the controller.
Firmware bug: Software in the controller may contain bugs that manifest under specific conditions.
Hardware defect: Capacitors, chips, memory – any controller component can fail.
Consequences
Array not recognized: Even though all disks are fine, without controller the system doesn't see them as RAID.
Metadata in controller: Some controllers store critical configuration information only in the controller, not on disks.
Solution
Replacement with compatible controller: Same model, ideally same firmware revision. Controller reads metadata from disks and array should be accessible again.
Professional recovery: If compatible controller is unavailable, array can be reconstructed virtually using specialized tools.
Prevention
- UPS against surges
- Document controller model and firmware version
- Have spare controller ready (for critical systems)
3. URE During Rebuild
What is URE
Unrecoverable Read Error – a read error that the disk cannot correct even after repeated attempts.
Why It Appears During Rebuild
Normal operation doesn't read all sectors. Some files haven't been opened in years. But rebuild must read every sector of every disk.
Sectors that haven't been read for years may be degraded. During rebuild, this is discovered for the first time.
Statistics
| Disk | URE rate | Probability during full read |
|---|---|---|
| Consumer 4TB | 10^14 | ~10-20% |
| Consumer 12TB | 10^14 | ~50-90% |
| Enterprise 12TB | 10^15 | ~5-15% |
Consequences for RAID
RAID 5: One URE during rebuild = rebuild failure. Array is unrepairable in standard way.
RAID 6: Tolerates one URE thanks to second parity. That's why RAID 6 is safer for large disks.
Prevention
- Use RAID 6 for large arrays and large disks
- Enterprise disks have better URE rate
- Regular scrubs (integrity checks) reveal URE early
4. Incorrect Rebuild After Disk Replacement
What Happens
IT technician sees "disk failed" and replaces disk. But replaces the wrong one. Or replaces the correct one, but initializes array instead of rebuild.
Typical Scenarios
Incorrectly labeled failed disk: System reports "Disk 3 failed". Technician pulls disk from slot 3. But slot numbering doesn't match software numbering. Pulled healthy disk.
Replacing multiple disks at once: "I'll replace all old disks while I'm at it." But replacing multiple disks simultaneously can trigger initialization of entire array.
Initialize instead of Rebuild: In management interface, "Rebuild" button is next to "Initialize" button. One restores data, the other deletes it.
Consequences
Loss of data that could have been saved. Sometimes complete, sometimes partial.
Prevention
- Take photos of state before replacement
- Double-check disk number
- Never change multiple disks at once
- Training for IT personnel
- Document procedures
5. Power Failure Without UPS
What Happens
Electricity fails in middle of operation. Data in write cache is not written. Metadata may be inconsistent.
Why It's Critical
Write cache: RAID controller has write cache – temporary memory where it writes data before storing on disks. During power failure, cache is erased.
Metadata: RAID maintains metadata about array state, stripe mapping, disk state. If metadata isn't updated atomically, it may be inconsistent.
Consumer vs Enterprise
Consumer controllers: Small capacitor to complete current write. Not enough to write entire cache.
Enterprise controllers: BBU (Battery Backup Unit) or FBWC (Flash Backed Write Cache) – battery or flash memory that preserves cache during power failure.
Consequences
- Lost data from cache
- Corrupted metadata
- Array in "foreign" or "offline" state
Prevention
- UPS for every server with RAID
- BBU/FBWC on enterprise controller
- Regular UPS and battery testing
6. Firmware Bug in Controller
Real-World Examples
HP Smart Array bugs: Some HP Smart Array firmware versions had bugs that could cause data loss under specific conditions.
Dell PERC issues: Problems with BBU, false positive disk failures.
Specific versions: Almost every manufacturer has historically had firmware versions that caused problems.
Why It Happens
RAID controller is a complex system. Software manages:
- Reading and writing to many disks
- Parity calculation
- Cache management
- Hot spare failover
- Error handling
In such complex code, bugs are inevitable. Most are caught during testing, but some slip through.
Edge Cases
Bugs often manifest under specific conditions:
- Full disk + specific write pattern
- Degraded rebuild + power failure
- Specific disk combination
Prevention
- Follow firmware update release notes
- Don't apply updates immediately after release (wait for feedback)
- Always backup before update
- "If it works, don't change it" (but have backup)
7. Human Error
Statistics
Studies show that 40-60% of RAID failures are caused by human error, not hardware.
Common Mistakes
Initializing array instead of rebuild: Buttons are next to each other. One click can delete everything.
Wrong configuration: Creating array with wrong stripe size, wrong RAID type, wrong disk order.
Incorrect disk order: After service, disks are inserted in different order. Array doesn't assemble correctly.
Formatting: "I thought I was formatting the other disk."
Removing "failed" disk: "It was glowing red, so I pulled it out." But that was warning, not critical.
Urgent steps during degraded state →
Prevention
- Training: Everyone working with RAID must understand basics
- Documentation: Written procedures for common operations
- Control mechanisms: Before deleting something, ask colleague
- Backups: When I make mistake, I have fallback
8. Aging – Simultaneous Failure from Age
What It Means
Disks purchased at the same time have similar lifespan. If operated under same conditions, they'll fail around the same time.
"Bathtub Curve"
Disk reliability follows bathtub-shaped curve:
- High mortality at start: Defective units fail early
- Stable period: Reliable operation
- Increasing mortality at end: Wear manifests
Disks from same batch enter final phase approximately simultaneously.
Why It's a Problem
For RAID 5 with 8 disks after 5 years:
- 1 disk fails (expected)
- You start rebuild
- During rebuild, 2nd disk fails (had same age)
- Data lost
Prevention
Staggered replacement: Don't replace all disks at once. Gradual replacement means disks of different ages.
Different batches: When purchasing, buy disks from different suppliers or at different times.
Proactive replacement: After 4-5 years, consider preventive replacement even if disks work.
SMART monitoring: Monitor SMART values. Reallocated Sector Count and Current Pending Sector predict failure.
What to Do When RAID Fails
1. STOP
Don't take hasty steps. Most damage occurs after initial failure through inappropriate intervention.
2. Document
- Screenshot of state (if possible)
- Which LEDs are lit and how
- Event logs
- What preceded failure
3. Don't Replace Disks Randomly
Without documentation and thoughtful plan, you can make situation worse.
4. Contact Expert
Professional diagnostics free. We'll determine what happened and what options exist.
What to do in degraded state →
FAQ
Can all RAID failures be prevented?
No. But risk can be minimized and you can be prepared for failure. Backups are the only real protection.
How often do RAID arrays fail?
Depends on many factors. Quality enterprise RAID with new disks, proper configuration and monitoring can run for years. Cheap NAS with consumer disks and no backups is a ticking time bomb.
Is RAID or backup better?
Both. RAID protects against disk failure (immediate outage). Backup protects against everything else (deletion, ransomware, fire, human error). One doesn't replace the other.
Need Help?
If your RAID array has failed, we can determine the cause and recovery options. Diagnostics is free.
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