To mount additional hard disk in Ubuntu in Contabo. You will need to have root privilege to do it.
Use fdisk to identified the disks in the system. Below is the link for the official guide
https://contabo.com/blog/mounting-additional-hard-disks-linux/

fdisk -l

If the Disk have been already formated then you can just create a directory and mount the drive mapped to the directory.

mkdir /storage

mount /dev/sdb1 /storage