6.3.3. Replacing a Faulty Device

To replace a particular device in a software RAID, first make sure it is marked as faulty by running the following command as root:
mdadm raid_device --fail component_device
Then remove the faulty device from the array by using the command in the following form:
mdadm raid_device --remove component_device
Once the device is operational again, you can re-add it to the array:
mdadm raid_device --add component_device

Example 6.3. Replacing a faulty device

Assume the system has an active RAID device, /dev/md3, with the following layout (that is, the RAID device created in Example 6.2, “Creating a new RAID device”):
~]# mdadm --detail /dev/md3 | tail -n 3
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
Imagine the first disk drive fails and needs to be replaced. To do so, first mark the /dev/sdb1 device as faulty:
~]# mdadm /dev/md3 --fail /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md3
Then remove it from the RAID device:
~]# mdadm /dev/md3 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1
As soon as the hardware is replaced, you can add the device back to the array by using the following command:
~]# mdadm /dev/md3 --add /dev/sdb1
mdadm: added /dev/sdb1