mdadm auto-read-only
If your array looks like this,
md1 : active (auto-read-only) raid10 sda2[0] sdd2[3] sdc2[2] sdb2[1] 7810048 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]
It’s very easily corrected by using mdadm to set the array as readwrite like this.
mdadm --readwrite /dev/md1
This will cause the array to resync. You can check it’s status like this.
cat /proc/mdstat