A disk recently died on my software raid array, and replacing the disk was a simple procedure. You will get some notification via your systems email regarding that a disk has failed. Usually you can periodically check the mdstat file and see if any disks have failed. You can check the mdstat file below.
The output should be similar to below.
You can see from up above that one of the disks has died. Going by the sequence of disks in the /proc/mdstat file. I can make out that /dev/sdg1 has failed ( I have gone a bit ahead on the sequence). I now need to remove /dev/sdg1 from the array. You do not need to stop the array to remove the disk. The command below to remove the disk is
Unless you have hot swappable disks, you need to shut down the system to swap the old disk out and replace it with a new disk. Once the system is rebooted. You can type the command to add the new disk the existing array.
This will take some while for the disk to resync with the existing array. To monitor the progress of the array, you just type in the following command.
You can ctrl-c to get out of this anytime. It wil update every ten seconds.