Removing a drive from a RAID array is sometimes necessary if there is a fault or if you need to switch out the disk.
For a device to be removed, it must first be marked as “failed” within the array. You can check if there is a failed device by using mdadm --detail:
sudo mdadm --detail /dev/md0
Output
/dev/md0:
Version : 1.2
Creation Time : Wed Aug 10 21:42:12 2016
Raid Level : raid5
Array Size : 209584128 (199.88 GiB 214.61 GB)
Used Dev Size : 104792064 (99.94 GiB 107.31 GB)
Raid Devices : 3
Total Devices : 3
Persistence : Superblock is persistent
Update Time : Thu Aug 11 14:10:43 2016
State : clean, degraded
Active Devices : 2
Working Devices : 2
Failed Devices : 1
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
Name : mdadmwrite:0 (local to host mdadmwrite)
UUID : bf7a711b:b3aa9440:40d2c12e:79824706
Events : 144
Number Major Minor RaidDevice State
0 0 0 0 removed
1 8 0 1 active sync /dev/sda
2 8 16 2 active sync /dev/sdb
0 8 32 - faulty /dev/sdc
The highlighted lines all indicate that a drive is no longer functioning (/dev/sdc in this example).
If you need to remove a drive that does not have a problem, you can manually mark it as failed with the --fail option:
sudo mdadm /dev/md0 --fail /dev/sdc
Output
mdadm: set /dev/sdc faulty in /dev/md0
If you look at the output of mdadm --detail, you should see that the device is now marked faulty.
Once the device is failed, you can remove it from the array with mdadm --remove:
sudo mdadm /dev/md0 --remove /dev/sdc
Output
mdadm: hot removed /dev/sdc from /dev/md0
You can then replace it with a new drive, using the same mdadm --add command that you use to add a spare:
sudo mdadm /dev/md0 --add /dev/sdd
Output
mdadm: added /dev/sdd
The array will begin to recover by copying data to the new drive.
