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I have a media server running Plex over on Hetzner, but am in no way a server pro.

I failed to realise that my two 2.0TB hard drives would be completely separate, so instead of 4.0TB of space in my main /home directory it's capped itself at 1.8 (which I only realised after uploading 1.8TB of data...)

Running df -h I get:

Filesystem Size Used Avail Use% Mounted on
udev 16G 0 16G 0% /dev
tmpfs 3.2G 13M 3.2G 1% /run
/dev/md2 1.8T 1.7T 4.4G 100% /
tmpfs 16G 8.0K 16G 1% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 16G 0 16G 0% /sys/fs/cgroup
/dev/md1 488M 183M 280M 40% /boot
tmpfs 3.2G 0 3.2G 0% /run/user/1000

And df /home tells me:

Filesystem 1K-blocks Used Available Use% Mounted on
/dev/md2 1905568740 1805456300 3292060 100% /

So my home folder is stored on md2, which is saying it's full.

But according to fdisk -l, I have another 1.8TB available on sdb:

Disk /dev/sda: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x8c0aea5f

Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 33556479 33554432 16G fd Linux raid autodetect
/dev/sda2 33556480 34605055 1048576 512M fd Linux raid autodetect
/dev/sda3 34605056 3907027119 3872422064 1.8T fd Linux raid autodetect


Disk /dev/sdb: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x9612d2cc

Device Boot Start End Sectors Size Id Type
/dev/sdb1 2048 33556479 33554432 16G fd Linux raid autodetect
/dev/sdb2 33556480 34605055 1048576 512M fd Linux raid autodetect
/dev/sdb3 34605056 3907027119 3872422064 1.8T fd Linux raid autodetect


Disk /dev/md1: 511.4 MiB, 536281088 bytes, 1047424 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/md2: 1.8 TiB, 1982545854464 bytes, 3872159872 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/md0: 16 GiB, 17163091968 bytes, 33521664 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

I've looked at walkthroughs on how to install LVM, as a possible method of creating a "combined drive" volume, but it's completely over my head.

Can anyone help me with a "for dummies" step-by-step guide on how to merge the two drives, so I can utilise the full 4.0TB for my /home directory? Preferably without losing the 1.8TB of data I've already uploaded there!

Revision:

mdadm --examine returns "mdadm: No devices to examine"

lsblk results:

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb 8:16 0 1.8T 0 disk
├─sdb2 8:18 0 512M 0 part
│ └─md1 9:1 0 511.4M 0 raid1 /boot
├─sdb3 8:19 0 1.8T 0 part
│ └─md2 9:2 0 1.8T 0 raid1 /
└─sdb1 8:17 0 16G 0 part
 └─md0 9:0 0 16G 0 raid1 [SWAP]
sda 8:0 0 1.8T 0 disk
├─sda2 8:2 0 512M 0 part
│ └─md1 9:1 0 511.4M 0 raid1 /boot
├─sda3 8:3 0 1.8T 0 part
│ └─md2 9:2 0 1.8T 0 raid1 /
└─sda1 8:1 0 16G 0 part
 └─md0 9:0 0 16G 0 raid1 [SWAP]

and cat /proc/mdstat:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sdb1[1] sda1[0]
 16760832 blocks super 1.2 [2/2] [UU]

md2 : active raid1 sdb3[1] sda3[0]
 1936079936 blocks super 1.2 [2/2] [UU]
 bitmap: 1/15 pages [4KB], 65536KB chunk

md1 : active raid1 sdb2[1] sda2[0]
 523712 blocks super 1.2 [2/2] [UU]

unused devices: <none>

SOLVED: Apparently it wasn't possible to combine drives without erasing everything and starting from scratch, so I started the Hetzner server in Rescue mode and followed the walkthrough here to setup Raid0 and LVM:

https://www.youtube.com/watch?v=cf7g-qDoUho

From there I reinstalled Plex and started uploading everything from scratch...