“Grow” the array down to two devices:
$ sudo mdadm --grow /dev/md1 --raid-devices=2 --backup-file /tmp/mdadmbackup
raid_disks for /dev/md1 set to 2
Show the new status:
/dev/md1:
Version : 1.2
Creation Time : Fri Sep 16 22:38:10 2011
Raid Level : raid1
Array Size : 1953510841 (1863.01 GiB 2000.40 GB)
Used Dev Size : 1953510841 (1863.01 GiB 2000.40 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Jul 4 18:27:02 2017
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : microserver:1 (local to host microserver)
UUID : e19da472:04736a59:6c1e25ec:c4a60ebe
Events : 20513
Number Major Minor RaidDevice State
3 8 49 0 active sync /dev/sdd1
2 8 17 1 active sync /dev/sdb1
References: