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)
  • parted –script /dev/sdc “mklabel gpt”
  • parted –script /dev/sdc “mkpart primary 0% 100%”
  • parted –script /dev/sdc “set 1 raid on”
  • # Repeat for /dev/sdd
• Add them both to the RAID1:
  • mdadm /dev/md0 –add /dev/sdc1 /dev/sdd1
• Fail one of the old drives:
  • mdadm /dev/md0 -f /dev/sda1
• Wait for sync to finish:
  • watch -n 60 cat /proc/mdstat
• Fail the other old drive:
  • mdadm /dev/md0 -f /dev/sdb1
• Wait for sync to finish:
  • watch -n 60 cat /proc/mdstat
• Remove the old drives from the array:
  • mdadm /dev/md0 -r /dev/sda1 /dev/sdb1
• Grow the array:
  • mdadm –grow /dev/md0 –size max
• Grow the filesystem:
  • resize2fs /dev/md0
  • # Or xfs_growfs, resize2fs, etc…

..and now I have a 14TB RAID1 instead of the initial 4TB.

Originally from; StackExchange