So I’ve had to replace a Seagate disk yet again and spent a frustrating amount of time on their website looking for a link to their warrenty replacement page >> http://www.seagate.com/support/warranty-and-replacements/

At least this time, I’m using Linux software RAID for a RAID1 mirroring configuration. When I determined there was a disk failure, I used the following mdadm commands to remove the bad drive:

# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda1[0](F) sdb1[2]
5139084 blocks [2/1] [U_]
md1 : active raid1 sda2[0](F) sdb2[2]
9841585344 blocks [2/1] [U_]
unused devices:

– Fail and remove all /dev/sdb partitions (/dev/sdb1, /dev/sdb2)
# mdadm --manage /dev/md0 --fail /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md0
# mdadm --manage /dev/md0 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1
# mdadm --manage /dev/md1 --fail /dev/sdb3
mdadm: set /dev/sdb3 faulty in /dev/md1
# mdadm --manage /dev/md1 --remove /dev/sdb3
mdadm: hot removed /dev/sdb3

– Shutdown and replace the bad disk (assuming you have been able to replace with the exact disk)
– Copy the partition table from the surviving disk
# sfdisk -d /dev/sda | sfdisk /dev/sdb

– Re-attach the partitions from /dev/sdb to the RAID1 mirrors:
# mdadm --manage /dev/md0 --add /dev/sdb1
# mdadm --manage /dev/md1 --add /dev/sdb2

You should now see the md devices syncing up by:
# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda1[1] sdb1[2]
5139084 blocks [2/1] [U_]
[======>.......] recovery = 49.3% ...

Once the sync completes, install grub on both the drives again:
# grub
grub> root (hd0,0)
grub> setup (hd0)
grub> root (hd1,0)
grub> setup (hd1)

Great reference pages:
http://serverfault.com/questions/483141/mdadm-raid-1-grub-only-on-sda
https://blogs.it.ox.ac.uk/jamest/2011/07/26/software-raid1-plus-grub-drive-replacement/