I run a small Samba server at home that I use to store high-fidelity music and photos. Right now it's based upon a RAID 1 mirrored pair which means I can only withstand a single disk failure before losing my data.
First, you'll want to inspect your current configuration carefully especially if you have an encrypted LUKS setup. It's easy to forget what in what order you created the array!
After making sure all drives are plugged in, we'll take a look at our current block devices.
# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 16.2M 1 loop /snap/node/2028 loop1 7:1 0 88.4M 0 loop /snap/core/6964 sda 8:0 0 55.9G 0 disk ├─sda1 8:1 0 512M 0 part /boot/efi ├─sda2 8:2 0 244M 0 part /boot └─sda3 8:3 0 55.2G 0 part ├─vg-root 254:0 0 17.7G 0 lvm / ├─vg-swap 254:1 0 3.9G 0 lvm [SWAP] └─vg-home 254:2 0 33.6G 0 lvm /home sdb 8:16 0 1.8T 0 disk ├─sdb1 8:17 0 200M 0 part ├─sdb2 8:18 0 1.8T 0 part └─sdb3 8:19 0 128M 0 part sdc 8:32 0 1.8T 0 disk ├─sdc1 8:33 0 200M 0 part ├─sdc2 8:34 0 1.8T 0 part └─sdc3 8:35 0 128M 0 part sdd 8:48 0 1.8T 0 disk └─sdd1 8:49 0 1.8T 0 part
Here you can see my internal hard drive (sda) and my external hard drives (sdb, sdc, and sdd).
sdd is the new drive we'll be adding.
I have a unique setup on my RAID array: I have used the entire disk instead of individual partitions. It makes it easier to add and remove drives without worrying about matching the partition table. If you used a specific partition, then recreate that partition table exactly upon the new drive before continuing.
# mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/disk/by-id/usb-WD_Elements_EXISTING_DISK_1-0:0 /dev/disk/by-id/usb-WD_Elements_EXISTING_DISK_2-0:0 [... warnings about being part of an existing array ... ] mdadm: array /dev/md0 started.
Next, add the new drive to your RAID array and check /proc/mdstat about the progress.
# mdadm /dev/md0 --add /dev/disk/by-id/usb-WD_Elements_NEW_DISK_1-0:0 mdadm: added /dev/disk/by-id/usb-WD_Elements_NEW_DISK_1-0:0 # cat /proc/mdstat Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] md0 : active raid1 sdd(S) sdc sdb 1953350656 blocks super 1.2 [2/2] [UU] [>....................] resync = 0.8% (16522496/1953350656) finish=1121.6min speed=28778K/sec bitmap: 15/15 pages [60KB], 65536KB chunk unused devices: <none>
From the output, we can see that the resync is beginning to the new disk. It will take a while -- especially these drives being connected over USB 3. The advantage of building your LUKS or LVM devices upon the base MD array is that you're now done!