if you do not have a secont drive, skip the first part and start with creating the new LV.

Extend a logical Volume Group [only if you have a second drive]

as we decided to use LVM we need to find the unused drive and extend our LVM group with this drive:

sudo lsblk

will help you to find the unused drive

here we would like to use the SSD /dev/sda

Now we create a physical volume (PV) on this drive:

sudo pvcreate /dev/sda

We find out the name of the volume group we would like to extend:

sudo vgdisplay

and we extend that group with the newly created PV

sudo vgextend roon-core2-vg /dev/sda

sudo vgdisplay

worked:

Add a new partition for music in the LV [start here if you do not have a second drive]

using all avialable space:

sudo lvcreate -n music -l 100%FREE roon-core2-vg

create the filesystem:

sudo mkfs.ext4 /dev/roon-core2-vg/music

create the director to mount the new file system to:

sudo mkdir /mnt/music

now edit the /etc/fstab to include:

/dev/mapper/roon--core2--vg-music /mnt/music ext4 defaults 0 2

sudo vim /etc/fstab
# now remount fstab
mount -a
sudo systemctl daemon-reload

and finaly allow the user roon to access all that newly created space:

sudo chown roon:roon /mnt/music
sudo chmod 700 /mnt/music
sudo chown -R roon:roon /mnt/music