Double disk failure on RAID 5: the most frequent scenario. Almost always recoverable.
This is the most common case among our recovery jobs: a RAID 5 array with one disk left in unhandled predictive failure for months, a second disk that dies, and a rebuild that fails because the first one was already at its limit. Power down, do not retry the rebuild, call us. Done correctly, the procedure recovers the data in most cases.
How you end up here.
- One disk in the RAID 5 array goes into predictive failure. The system flags it, but nobody acts on the alert — it happens.
- Months later, a second disk fails outright. The array goes degraded.
- The system (or the administrator) starts a rebuild onto a hot-spare or a replacement disk.
- During the rebuild — an operation that stresses every remaining disk — the first disk (the one in predictive failure that was never replaced) gives out for good.
- Array failed. Volume will not mount. Red alert.
The first 10 minutes matter.
- Do not reboot repeatedly.
- Do not start new rebuilds.
- Do not run chkdsk / fsck on the volume.
- Do not pull the disks and change their physical order.
- Photograph the current state. Power down. Call.
Cloning, logical reconstruction, filesystem recovery.
- Bit-by-bit cloning of every disk, failed ones included. On marginal disks we use aggressive tools that read difficult sectors with multiple retries.
- Identification of the slot order and the RAID parameters from the DDF metadata.
- Virtual reconstruction of the array on the clones — never on the original disks.
- Filesystem recovery (NTFS, ext4, XFS, VMFS) on the reconstructed array.
- Data returned on new media supplied by the customer; original disks returned in a sealed bag.
The questions we get most often.
How long does it take to recover a double disk failure?
From a few days to 2 weeks, depending on disk capacity and the extent of the physical damage. Cloning is the slowest part (up to 24-48h per enterprise-class disk). Logical reconstruction and filesystem recovery come after that.
How likely is a full recovery?
In our track record, the large majority of double-disk RAID 5 cases with disks that are even only partially readable end in a complete or near-complete recovery. We give realistic figures for your specific case after the first inspection of the disks.
Is it expensive?
This is specialist work, and the cost reflects the lab time and the tools required (PC-3000, cleanrooms where needed). The preliminary diagnosis always gives you a cost range before the work starts. An honest comparison: recovery costs are typically a fraction of the cost of rebuilding the lost data by hand (where that is even possible), and well below the cost of prolonged downtime.