mdadm --manage /dev/md3 --add /dev/sda1
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md0 : inactive sdd2 sdd1(S)
md3 : active raid1 sda1 sdb1 sdc1(F)
943730240 blocks [2/1] [_U]
[>....................] recovery = 4.9% (46270656/943730240) finish=217.0min speed=68900K/sec
Important Install GRUB on the new drive or you won't be able to boot from it:
This is critical because you won't be able to boot off the new drive if the other fails (you'll likely just see a flashing cursor after your system posts and will wonder what the issue is).
Change /dev/sda to whatever the new drive that you added is (and I do mean sda and not sda1).