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RAR / Every level and layout / RAID 5

RAID 5 · left-symmetric · right-asymmetric · 64K · 256K · PERC · Smart Array · mdadm

RAID 5 data recovery. One parity block per stripe, one failure tolerated, and a rebuild that reads every sector to find the second.

RAID 5 stripes data across every member and rotates one block of parity through the stripes, so that any single member can be computed from the others. It tolerates one failure, and it is the level that arrives here most, because the thing that follows one failure is a rebuild, and a rebuild reads every sector of every survivor to compute the missing member. On drives bought together, that is where the second failure lives: an unrecoverable read error on a survivor part-way through, a drive marked predictive that the rebuild reads from end to end, or a member that dropped out weeks ago and was forced back in with stale data. The bench does the same arithmetic on images, which cannot stall, and it needs four things the controller knew and may no longer say: the order, the stripe size, the rotation, and the offset. A RAID 5 of two to four members is £500 + VAT upwards after the free look, fixed in writing, 5–10 days at the bench; five or more from £1,250 + VAT.

Free first lookOne fixed figure in writingNo data, no bill on most jobsReturn postage paid

Rather talk it through? An engineer answers the bench line
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Power down. Label every slot before a drive comes out. Do not rebuild, do not force a drive online, do not import or clear a foreign configuration, do not initialise, and do not run chkdsk, fsck, zpool import -F or mdadm --create on the members. A rebuild reads every sector of every survivor and writes to the replacement; each of the commands writes to the drives. On images, all of them are reversible. On the originals, none of them is.

RAID 5 symptoms, and how long each gives you.

Not listed? Describe it on the form →
Degraded, one member outReadable now, no margin. Copy what matters; image before any rebuild.Do not delay
Rebuild stalled at n per centA survivor's unrecoverable read error. Power down; the first-dropped drive still holds those stripes.Do not delay
Failed or offline after a second drive droppedEvery member imaged; the set reassembled from images.Do not delay
Foreign configuration after a controller swapThe description is on the drives. Do not clear it.Do not delay
Volume mounts but files are corrupt after a rebuildA stale member forced online, or a puncture. Stop writing.Do not delay
Recreated with the same settings; volume emptyMetadata rewritten; data usually intact. Do not initialise.Do not delay
Packing it and posting it: power down, photograph the controller's screen or export its log, and write the slot number on each drive with a marker before it comes out of its carrier. Send every member of the set, including the one that failed first; it holds the stripes the rebuild never reached. Each drive travels in an anti-static bag inside its own padding, in a box with nothing able to move. Send the controller if it was replaced or holds an encryption key; otherwise it stays. Insure the parcel for what the data is worth, use a tracked service, and know that the posting address is not printed anywhere on this site; it arrives by email in reply to the form, with a booking sheet to print; the sheet inside the parcel is what matches it to your enquiry when it is opened. Or hand the sealed parcel in at the nearest of ten drop-off points, your name on the outside and the sheet inside; say where you are on the form and it comes by email. We pay the postage home either way. The whole of it is written up on the guide to packing and posting.

What it tolerates, what fails it, and what the bench has to find.

Rotation and symmetryParity moves through the stripes from the last disk backwards (left) or the first disk forwards (right), and after the parity block the data either restarts on the next disk (symmetric, mdadm's default left-symmetric) or always starts on disk 0 (asymmetric). Four combinations, plus parity-first and parity-last, plus the delayed-parity variant recovery tools list for Smart Array. Guess it wrong and every stripe reads as garbage past the first parity block.
The second failureA survivor that returns an unrecoverable read error during the rebuild either fails the set, or on a PERC is skipped, which is a puncture. The drive that dropped out first still holds those stripes, which is why it should travel with the set and never with the courier.
The stale memberA drive that dropped weeks ago and was forced online carries data the others have long since overwritten. The controller's event counts, or mdadm's, say which member is current; on the bench parity across the members finds the stale one where the counts are gone.
The write holeA power cut between the data write and the parity write leaves a stripe whose parity disagrees with its data. A consistency check afterwards rewrites parity from the data, which is right; a rebuild afterwards computes a member from the wrong parity, which is not.

What a reconstruction has to determine.

Describe yours to us →
Unknown Where it is read from What goes wrong when it is guessed
Member orderDDF or RIS metadata; mdadm superblock roles; failing that, file-system anchors and entropyEvery stripe past the first reads from the wrong disk
Stripe sizeMetadata; PERC defaults to 64KB, mdadm to 512K; failing that, entropy boundariesFiles larger than one stripe interleave with other files
Rotation and symmetryMetadata; failing that, where parity XORs to zeroData reads correctly until the first parity block, then not
Data offsetMetadata; the partition table or superblock on member 0The volume's start is missed and the file system will not mount
Stale memberEvent counts; parity mismatch patternOld data assembled into new files; corruption spread across the volume

From the parcel arriving to your files going back.

Work we have closed →
01

Logged the day it lands, and the first look costs nothing Free

A case number goes on the parcel and a number on every member the day it is opened, matched to the slot you wrote on it. Each member goes on the imager its interface needs, never on a controller, and its metadata is read before a sector is: the level, the order, the stripe size, the event counts that say which member is current and which dropped out first. An engineer settles what has happened to the set and how much of it can honestly be read back. Back to you come two things together: a straight note of what is liftable and what is not, plus one figure, fixed and written down. Accept it, or decline and owe us nothing.

Nothing to pay for lookingA single figure, put in writingNo rebuilds, no imports, no initialise
02

Every member imaged, including the one that failed first

Every drive in the set is imaged sector by sector, weak areas last, on hardware that controls every retry, with a map of what could not be read kept for each. Members with failed heads go to the clean bench first; that is the drive site's work and the same bench does it. The drive that dropped out first is imaged too, because it still holds every stripe written before it dropped, and a rebuild that stalled part-way never reached them.

Sector by sector, weak areas lastThe first-dropped member included
03

The geometry, from the metadata or from the parity

Where the controller's metadata survives on the members, the order, stripe size, parity rotation and data offset are read from it. Where it was cleared or overwritten, they are recovered from the data: parity across the members at the same offset should XOR to zero on a consistent stripe, which confirms the level and finds the stale member; where parity lands says the rotation; entropy at stripe edges and the file system's own anchors give the stripe size, the order and the start.

Metadata first, parity secondOrder, stripe, rotation, offset
04

Assembled in software, and repaired on the virtual volume

The set is put together from the images in software, with nothing written to any of them: the current members in, the stale member used only to fill holes a survivor could not give. The file system is checked and repaired on a copy of the virtual volume, VMFS and CSV volumes opened and the virtual machines' disks extracted, databases repaired where they need it. The originals are not touched again.

Nothing written to the imagesVirtual machines and databases opened
05

You see the file list before you pay

What was recovered is listed for you first, and only then does a bill exist. Approve the list and it is invoiced; turn it down and it is not — and where nothing has come back, most jobs carry no charge at all. Recovered data travels home on fresh media bought in for your job, with the postage at our end. Your case is not closed until you have opened the files on a machine of your own.

No charge until you accept the figureFresh media, supplied with the job5–10 days at the bench

What arrives most often

  • The first-dropped drive is not rubbish. It holds every stripe written before it dropped, and a rebuild that stalled never reached them. Send it.
  • Do not recreate the array to see whether it comes back. A recreate with the same settings rewrites the metadata and, on most controllers, starts an initialisation.
  • Punctures are the PERC's version of skipping. The set reports Optimal with holes in it. Dell's own cure is to delete and recreate; the bench images first. The puncture itself is on the drive site.
  • Say whether the level was known. A set built ten years ago by a contractor who has left is still recoverable; the metadata usually remembers what the owner does not.

How much comes back: from a RAID 5 with one failure and a stalled rebuild, usually all or nearly all of it, with the stripes the rebuild could not compute filled from the first-dropped drive. From a set rebuilt into a second failure and then recreated and initialised, less.

One job, followed all the way through.

UK · RAR-2026-0601JOB LOGGED ✓

A six-drive RAID 5 on a PERC H730, one member failed, a second dropped at 61 per cent of the rebuild, and the virtual disk marked Failed with a firm's file server on it

The owner powered down and sent all six drives, including the first to fail. The two dropped members were imaged, one after a head swap on the clean bench and the other slowly with its weak areas last, and the four survivors directly. The DDF metadata on the drives gave the order, a 64KB stripe and the rotation; parity across the images confirmed it and identified the stale stripes on the first-dropped drive, which were used only where the second-dropped drive could not read. The NTFS volume mounted from the virtual array and the file server's shares were copied out.

99.98% of the volume recovered8 days here, and back by post
Illustrative example — replace with a genuine case

What helps, and what harms.

Do this much first

  • Power down and label the slots
  • Send every member, including the first to fail
  • Export the controller log or photograph the screen
  • Tell us the controller, the stripe size if known, and what was tried

What sets us back

  • Rebuilding with a predictive-failure survivor
  • Forcing the first-dropped drive online
  • Recreating the array to see whether it comes back
  • Running chkdsk or fsck on the degraded volume

Questions answered before you commit.

Can a RAID 5 with two failed drives be recovered?

Usually, if the two drives can be imaged. The one that failed first holds every stripe written before it dropped; the one that failed during the rebuild usually reads on the bench with its bad areas last. The set is reassembled from the images with the stale stripes used only to fill holes.

Is RAID 5 safe on large drives?

Less than it was. A rebuild of four 8TB drives reads 24TB from the survivors, and the rated unrecoverable-read-error figures make one error over that read likely on paper. The calculator shows the estimate, with the caveat that the rating is a ceiling and not a measured rate.

What if the array was recreated with the same settings?

The metadata was rewritten and the data usually was not, unless a full initialisation followed. The bench reads the geometry from the data and assembles the set from images. Do not initialise, and do not let a background initialisation run.

What does it cost?

£500 + VAT upwards for a set of two to four members after the free look, fixed in writing; five or more from £1,250 + VAT. On most jobs no data means no bill.

How long does it take?

5–10 days at the bench for a set of two to four; 10–15 days at the bench for larger sets.

Begin here if yours is doing the same thing.

Server data recovery →VMware data recovery →Rebuild failed: a second drive went during it →Virtual disk degraded →Dell PERC →HPE Smart Array →RAID calculator →

Nothing gets worse while it is powered down.

Looking at it is free. Back comes a list of what opened and what did not, together with a single price to finish, set down in writing while you are still free to say no. On most jobs an invoice only follows the data. Until that list reaches you, leave the server off and the drives in their slots.

0800 6890668

Price my jobPacking and posting