==== Grow MD RAID-1 Array ====
You can grow a RAID1... if you replace both disks. I just did, it worked like this:
  * Started with 2x4T drives (sda1 and sdb1) in a working RAID1 array (md0)
  * Bought two new 16T drives and plugged them in (sdc and sdd)
  * Partition them with full drive partitions (sdc1 and sdd1)
      * <wrap hi>parted --script /dev/sdc "mklabel gpt"</wrap>
      * <wrap hi>parted --script /dev/sdc "mkpart primary 0% 100%"</wrap>
      * <wrap hi>parted --script /dev/sdc "set 1 raid on" </wrap>
      * # Repeat for /dev/sdd
  * Add them both to the RAID1: 
      * <wrap hi>mdadm /dev/md0 --add /dev/sdc1 /dev/sdd1</wrap>
  * Fail one of the old drives: 
      * <wrap hi>mdadm /dev/md0 -f /dev/sda1</wrap>
  * Wait for sync to finish: 
      * <wrap hi>watch -n 60 cat /proc/mdstat</wrap>
  * Fail the other old drive: 
      * <wrap hi>mdadm /dev/md0 -f /dev/sdb1</wrap>
  * Wait for sync to finish: 
      * <wrap hi>watch -n 60 cat /proc/mdstat</wrap>
  * Remove the old drives from the array: 
      * <wrap hi>mdadm /dev/md0 -r /dev/sda1 /dev/sdb1</wrap>
  * Grow the array: 
      * <wrap hi>mdadm --grow /dev/md0 --size max</wrap>
  * Grow the filesystem: 
      * <wrap hi>resize2fs /dev/md0</wrap>
      * # Or xfs_growfs, resize2fs, etc...
..and now I have a 14TB RAID1 instead of the initial 4TB.

Originally from; [[https://unix.stackexchange.com/questions/665389/mdadm-adding-a-new-hard-disk-to-an-existing-raid1|StackExchange]]