There are three general reasons for a RAID failure.
- Failure of one of the RAID member disks;
- Failure of several RAID member disks;
- Other RAID failures not associated with the member disks (operator error or controller failure).
RAID 5 failures
RAID 5 can fail due to any of the mentioned above reasons. However, RAID 5 is a redudant array which means it can survive failure of one of the
member disk. If failure of your RAID 5 affects only one member disk then you are lucky and can easily get your data back.
In case of multiple disk failures there is zero chance of recovering the array data.
Failure of one of the RAID 5 member disks
You can notice that one of the member disks has failed if:
- For a NAS, the LED corresponding to the damaged disk turns red;
- Drivers or RAID 5 controlling software produce the message that the disk has failed.
In these situations all you need to do is to replace a failed disk and start a RAID rebuild.
Failure of several RAID 5 member disks
You can realize that several RAID 5 member disks have failed based on the following signs:
- For a NAS, the LEDs corresponding to the failed disks turn red.
- The array is not accessible; however, it seems that controller works properly.
Since RAID 5 is capable to survive single member disk failure only,
then for multiple disk failures it is impossible to recover data that was on the failed disks.
However, you can try to recover data which is contained on the remaining disks.
Files can be restored in the following cases:
- A file is located on a good disk and a file size is less than a block size.
- A file occupies several consequent blocks located on the good disks.
Other RAID 5 failures not associated with the member disks
Such RAID 5 failures include operator errors, controller failures, or RAID 5 controlling software failures.
In this case RAID 5 configuration metadata is lost, but the member disks are working properly.
With these failures, it is possible to recover data from RAID 5.
First, you should determine the array configuration parameters and then recover data from the array using any data recovery software.
RAID 5 configuration includes:
- number of member disks,
- disk order, along with what disk was the first in the array,
- block size,
- start offset on the disks,
- parity position and rotation.
The detailed instructions on how to recover RAID 5 are provided on the RAID 5 recovery
RAID 1E failures
Although RAID 1E is not a typical array layout, it is subject to the same types of failure as more common arrays are,
such as member disk failures or an operator error.
But not all the RAID 1E failures lead to data loss, for example in case of a single disk failure you just need to replace the disk and
rebuild the array.
RAID 1E can even survive a multiple disk failure given that the failed disks were nonadjacent. For more information about RAID1E layout,
please refer to RAID 1E page.
Failure of RAID 1E member disks
RAID 1E requires a minimum of three disks. RAID 1E can survive a failure of one member disk or any number of nonadjacent disks.
In this case to recover the array you need to disconnect failed disk(s), connect good disk(s) and start to rebuild as it is described in a controller manual.
Controller failure or operator error
In such a situation all the member disks are OK, but array parameters (e.g. block size and disk order) are lost.