You can create either a software RAID, or an LVM logical volume spanning two disks. Notice that by simply aggregating the disks, you also aggregate the chances of failure: once your disks are grouped in a virtual 4 TB drive, if either of these fails you'll lose everything.
From your lsblk
output, we see that your sdb drive is in use (as the system drive). Therefore you can't aggregate it to the other drive: you can only aggregate unused disks or partitions.
So your only option is to partition your /dev/sda
, make a filesystem on it, and mount it somewhere like /mnt
. All of this is easily done in Webmin from the "disk partition" tool in the "Hardware" subsection. Here is the way to do it from the command line:
# create the partition table (GPT)
sudo parted /dev/sda mklabel gpt
#create a partition spanning the whole disk
sudo parted /dev/sda mkpart 1 ext4 1 -1
#create a filesystem on the new partition
sudo mkfs.ext4 /dev/sda1
#create a mountpoint
sudo mkdir /mnt/disk
#mount the disk
sudo mount /dev/sda1 /mnt/disk
#edit /etc/fstab and add a line like this
/dev/sda1 /mnt/disk ext4 defaults 0 0