I referenced these instructions to remind me how to replace a drive.
In my case the output of mdstat looks like this:
# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda2[0] sdb2[1]
33615936 blocks [2/2] [UU]
md1 : active raid1 sda3[2](F) sdb3[1]
2096384 blocks [2/1] [_U]
md0 : active raid1 sda1[0] sdb1[1]
128384 blocks [2/2] [UU]
unused devices:
So I have three partitions on
keep reading Replacing failed software raided drive
