The output of /proc/mdstat is as follows :
Personalities : [raid1]
read_ahead 1024 sectors
md0 : active raid1 sdb1 sda1 [2/2] [UU]
* The [UU] part above specifies that both the partitions on the Raid Devices are up.
* If you get [_U] in place of [UU] then the First HDD is down and raid is booting linux through the second HDD. Fix your first HDD problem and the recreate RAID.
* [U_] indicate the Second HDD is down.
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