If you are happen to have a failed hard drive on your software RAID array, follow the following steps to successfully reconstruct the data on new drive.
in my case I had RAID 1 Array, and one of the drives ^&*( up, I had to buy new HDD.
# cat /proc/mdstat
Letter F will show next to the failed device.
Personalities : [raid1]
md0 : active raid1 sdc[1] sdb[2](F)
8388544 blocks [2/1] [_U]
Recovery
~~~~~~~~
1. use -r option to remove the device from the RAID Array.
# mdadm /dev/md0 -r /dev/sdb
mdadm: hot removed /dev/sdb
2. Insert your new HDD to the system, then use -a option to add your new drive to the RAID Array.
# mdadm /dev/md0 -a /dev/sdb
mdadm: re-added /dev/sdb
3. Watch your progress using watch cat /proc/mdstat
# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb[0] sdc[1]
8388544 blocks [2/1] [_U]
[>....................] recovery = 4.3% (366592/8388544) finish=5.8min speed=22912K/sec
Good Luck!