mdadm "auto-read-only" Linux Software RAID solution
If you have the "(auto-read-only)" beside an array I have no idea why that happens but it is easy to fix.
Just run "mdadm --readwrite /dev/md1" (rename md0 to the device with the problem and it will begin to resync.
md1 : active (auto-read-only) raid1 sdb2[0] sda2[1]
19534976 blocks [2/2] [UU]
resync=PENDING
md0 : active raid1 sda1[2] sdb1[0]
488287488 blocks [2/1] [U_]
[>....................] recovery = 1.9% (9341504/488287488) finish=302.1min speed=26421K/sec
unused devices: <none>
# cat /proc/mdstat
# mdadm --readwrite /dev/md1
# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sdb2[0] sda2[1]
19534976 blocks [2/2] [UU]
resync=DELAYED
md0 : active raid1 sda1[2] sdb1[0]
488287488 blocks [2/1] [U_]
[>....................] recovery = 1.9% (9677184/488287488) finish=302.5min speed=26368K/sec
unused devices: <none>