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06 Numbers and Powers

RAID 5 Data Recovery: What Works

Card No. 06·16220 · Updated · Standard formula, shown below

Data loss risk 33.3 %
Usable capacity
75.0 %
Result
Number of drives in array
4
You entered
Failed drives
1
You entered
You enter
Number of drives in array, Failed drives
You get
Data loss risk, Usable capacity
Rounding
1 dp

RAID 5 data recovery is possible in most cases, but the success rate depends heavily on how many drives have failed and whether you act before rebuilding. If only one drive has failed, the array can still function and data can be recovered by replacing the failed drive or using recovery software; if two drives have failed, professional recovery is often required.

Formula

The calculator estimates the percentage of the array’s storage that is at risk of data loss given the number of drives and how many have failed. RAID 5 stripes data and parity across all drives, so it can survive one drive failure without losing data. The formula is:

Data loss risk (%) = (Failed drives / (Total drives – 1)) × 100

Where:

  • Total drives – the number of physical drives in the RAID 5 array (minimum 3).
  • Failed drives – the number of drives that have failed or are missing.

The denominator (total drives – 1) represents the number of drives that can fail before data loss becomes certain. For example, with 4 drives, you can lose 1 drive safely, so the risk is 1/3 = 33% when one drive has failed.

Worked Example

Using the default values: 4 drives total, 1 failed drive.

  1. Calculate the denominator: 4 – 1 = 3
  2. Divide failed drives by denominator: 1 / 3 = 0.3333
  3. Multiply by 100: 0.3333 × 100 = 33.3%

The calculator shows 33% risk, meaning there is a one-in-three chance that the next drive failure will cause complete data loss. The usable capacity is (4-1)/4 = 75%, which is the typical overhead of RAID 5.

How RAID 5 Works

RAID 5 (Redundant Array of Independent Disks level 5) uses block-level striping with distributed parity. Data is split into blocks and written across all drives in the array. For each stripe of data blocks, one parity block is calculated (usually using XOR) and stored on a different drive. This parity block allows the array to reconstruct the data if one drive fails.

Because parity is distributed across all drives, no single drive is a dedicated parity drive (unlike RAID 4). This improves performance because write operations are spread across all drives. The usable capacity is (N-1) × drive size, where N is the number of drives. For example, four 1TB drives give 3TB of usable space.

Common Causes of RAID 5 Failure

RAID 5 arrays fail for several reasons, and understanding the cause helps determine the recovery approach:

  • Multiple drive failures – the most common cause. If two drives fail before the first is replaced, data is lost.
  • Controller failure – the RAID controller card can die, making the drives inaccessible even though they are fine.
  • Human error – accidentally deleting a volume, reinitializing the array, or pulling the wrong drive.
  • Power surges – can corrupt the RAID metadata or cause drives to drop offline.
  • Firmware bugs – some controllers have known bugs that can corrupt the array.

Recovery Options

When a RAID 5 array fails, you have two main paths:

Software Recovery

If only one drive has failed and the array is still readable, you can often recover data using RAID recovery software. These tools scan the drives, rebuild the array in software, and extract files. This works best when the controller is still functional and the failed drive is not physically damaged. Popular tools include R-Studio, ReclaiMe, and UFS Explorer. They can also create disk images to work from, reducing further risk.

Professional Recovery Services

If two drives have failed, the drives have physical damage (clicking, not spinning), or the controller is dead, you need a professional data recovery lab. These labs have clean rooms, specialized equipment, and expertise in reconstructing RAID arrays from raw drive images. They can often recover data even from severely damaged drives. Costs vary widely, but expect to pay hundreds to thousands of dollars depending on the number of drives and damage.

How to Choose a Recovery Method

Consider these factors when deciding:

  • Number of failed drives – one failed drive: try software first. Two or more: go professional.
  • Physical condition – if drives make unusual noises or are not detected, do not power them on; seek professional help.
  • Data importance – if the data is critical and you cannot afford loss, professional recovery is safer.
  • Budget – software is cheaper (often $50-$200) but only works in ideal conditions.
  • Time – software can take hours to days; professional labs may take days to weeks.

Prevention and Safety Tips

RAID 5 is not a backup. It protects against a single drive failure, but not against multiple failures, controller failure, or accidental deletion. To protect your data:

  • Always keep a separate backup of critical data, preferably offsite.
  • Monitor drive health with SMART tools and replace drives before they fail.
  • Label drives clearly and keep a record of the array configuration (order, stripe size, etc.).
  • Use a UPS to prevent power surges.
  • When a drive fails, replace it immediately and rebuild the array.

Frequently Asked Questions

Can I recover data from a RAID 5 with two failed drives?

Yes, but it is much harder. With two failed drives, the array is broken and you cannot simply rebuild. Professional recovery services can often reconstruct the data by analyzing the remaining drives and using the parity information, but success is not guaranteed. The longer you wait, the lower the chance, so act quickly.

How long does RAID 5 recovery take?

Software recovery can take anywhere from a few hours to several days, depending on the size of the drives and the amount of data. Professional labs typically take 3-10 business days, but urgent service may be available for an extra fee.

Is RAID 5 still worth using?

RAID 5 is still used, but with large drives (8TB and above), the risk of a second drive failing during a rebuild is higher because rebuild times are long. Many now prefer RAID 6 (which tolerates two failures) or RAID 10. However, RAID 5 remains a good balance of performance, capacity, and redundancy for small arrays.

What should I do immediately after a drive fails?

Do not panic. Do not rebuild the array automatically if you are unsure. First, power off the system to prevent further stress on the drives. Then, assess the situation: check which drive failed, and if you have a backup, you can rebuild. If not, consider using recovery software or a professional service before doing anything else.

Can I use recovery software on a RAID that is not detected by the OS?

Yes. Most RAID recovery software can scan raw drives and detect the RAID parameters (stripe size, order, parity rotation) even if the controller is not recognized. You may need to connect the drives to a different computer or use a hardware RAID controller in JBOD mode.

Related

Formula used
in t | Number of drives in array | - | 4 | 3 | 16
in f | Failed drives | - | 1 | 1 | 2
out p | Data loss risk | % | (f / (t - 1)) * 100 | 1
out usable | Usable capacity | % | (t - 1) / t * 100 | 1

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

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