Problem: One of the disks in your software RAID-1 array has failed.
Solution:
Check the status of your software raid array:
[root@server] mdadm --misc --detail /dev/md0
Remove the failed disk from the array set. Be careful not to remove the wrong one!
[root@server] mdadm --manage /dev/md0 --remove /dev/sdb1
Physically replace the faulty disk unit with a replacement which is identical to, or larger than the failed disk. Partition the new disk so that the partitions are equal to, or larger than, the size of the partitions on the healthy disk using fdisk.
Add the new disk to your software RAID-1 array:
[root@server] mdadm --manage /dev/md0 --add /dev/sdb1
The array should rebuilt automatically, but you can monitor its progress with:
[root@server] mdadm --misc --detail /dev/md0