Add disk back into a Linux MDADM software raid

I have a finicky disk on my home server that always drops from its RAID5 sofware raid.  The good thing for me is this RAID is one I use for low importance, bulk data, like OS ISO's and other stuff I can easily re-download if needed.  If you have a disk that keeps dropping from your RAID, I would recommend replacing the disk with one that is not so grumpy, and/or making sure you backups are in good shape.

However, I find that every now and then, usually after a reboot, this grumpy disk will eject itself from my RAID, so I have to re-insert it, and rebuild my array. Here is my "cookbook recipe" that I use to get it back. This post was mostly written for me, since I usually forget what commands I use, so I figured I would dump there for everyone else as well.  Lest get started:

First, I need to find out which disk ejected, so run the mdadm command to get the details:

mdadm --detail /dev/md127

You might see something that looks like this:

Screen Shot 2014-10-05 at 9.54.28 PM










I then use fdisk to find the disk that went missing:

 fdisk -cul

Now that I have found my disk, I can reinsert it into the array and let the rebuild begin:

 mdadm --manage /dev/md127 --add /dev/sde1

The disk should be added back to the array, and should now be rebuilding. You can check the status of the rebuild a few different ways. The way I prefer is by watching /proc/mdstat. This command works great:

watch cat /proc/mdstat

You can also get the details by running:

mdadm --detail /dev/md127

There are a couple of ways you can speed up the rebuild process a few different ways. Be sure to check my post on speeding up the rebuild here: Helpful tips to speed up a linux software RAID rebuild

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