Hotplug
From Linux Raid Wiki
HW issues of the disk hotplugging are described in the Hotswap chapter of theHardware issues page.
The Linux RAID supports hotplug operations fully from Hot-unplug branch of the mdadm version 3.1.2.
mdadm versions < 3.1.2
All previous versions of mdadm were unable to remove disconnected or dead devices from md arrays. The disconnected devices were displayed as "Failed spare" components of arrays and were seizing the device name forever. Example:
Let's have RAID1 composed of /dev/sda1 and /dev/sdb1
If we hot-unplugged the disk /dev/sda, the /proc/mdstat would display: (to be finished)
mdadm versions > 3.1.2
(to be finished)