Hello bourka,
the easiest would be to mount your additional volume (block storage) on the /wp-content/uploads directory.
To do so, move your wp-content uploads folder elsewhere :
cd /var/www/yoursite.tld/htdocs/wp-content/
mv uploads /var/www/yoursite.tld/
Then create an empty uploads directory
mkdir -p /var/www/yoursite.tld/htdocs/wp-content/uploads
Format your volume :
sudo mkfs.$FILESYSTEM /dev/disk/by-id/scsi-0HC_Volume_1251557
Mount your volume on the uploads directory
mount -o discard,defaults /dev/disk/by-id/scsi-0HC_Volume_1251557 /var/www/yoursite.tld/htdocs/wp-content/uploads
Add the mount to your ftstab
echo "/dev/disk/by-id/scsi-0HC_Volume_1251557 /var/www/yoursite.tld/htdocs/wp-content/uploads $FILESYSTEM discard,nofail,defaults 0 0" >> /etc/fstab
And mv back your uploads folder content
mv /var/www/yoursite.tld/uploads/* /var/www/yoursite.tld/htdocs/wp-content/uploads/