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I have a degraded RAID1 array running on Debian 7. I am trying to add another disk to the degraded array, but mdadm forgets the existence of the other disk at reboot. The array devices are /dev/sda1 and /dev/sda2 (two ext4 formatted 2TB partitions).

My problem is that the array will rebuild with /dev/sda1 and /dev/sdb1 but on restart, only /dev/sda1 will be included in the array (it will revert back to a degraded state). mdadm -D /dev/md0 doesn't indicate anything about /dev/sdb1 at all - just the original degraded /dev/sda1 configuration. But if i don't restart, the array sits there quite happily.

I have no idea where I'm going wrong! Why is mdadm "forgetting" that I have added /dev/sdb1 to the array?

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