2. Now we have enough information and we can try to re-assemble the raid and make it active first. This can be done by command “mdadm –assemble <MD_Name> <disks>”
Check above example, we use four disks (sdb/sdc/sdd/sde) to reassemble the raid. And it can be mounted successfully and the data is still there.
This is not the end. The raid is still in degraded mode since the faulty disk is not be replaced with new disk.
3. Replace the fault disk and replace it with a new disk. This step should be done very carefully. Make sure you don't’ replace wrong disk. If so, it might be a disaster.
4. Now we assume that the new disk is already in place and it’s sdf. Now we’ll add it to the raid and the raid will be re-built. Command “mdadm <MD_Name> -a <disk_name>” will do this.
See above example, after sdf is successfully added to md1, you can use command “cat /proc/mdstat” to check the current re-built progress. After it’s done, we can say that the problem is finally resolved.