When a RAID Array Refuses to Start

From Zanecorpwiki

An array will refuse to start unless all devices are present and clean. This means that on reboot, even workable arrays (3/4 RAID-5 devices present and ready) won't start. The tricky bit is that the RAID device (md0, md1, etc.) will have been created and needs to be stopped even though it's not active:

mdadm --stop /dev/md0

You can then assemble the array with:

mdadm --assemble -vv /dev/md0 /dev/sda3 /dev/sdb3 /dev/sdc3 /dev/sdd3

Adjusting the device references as necessary.

If an array has faults, such as with event overload, you may need to force the assembly. This is a last resort and since you're essentially saying "Ignore the errors", there's no guarantee the assembled array won't be corrupted.

If there is a bad drive in the array, replace it and add the new volume/partition:

mdadm /dev/md1 -a /dev/sdd3