I'am running Ubuntu 16.04 in Virtualbox on a Windows 7 host. I have created 3 other partitions from the virtualbox, and when I run sudo fdisk -l I get:

So I understand that /dev/sda is my master partition and /dev/sdb, /dev/sdc, /dev/sdd are my slave partitions.
Now my question is: how do I mount these partitions and copy files from the master partition to any of the other partitions.
I am primitive to ubuntu so please help me by being clear in each step.
Thank you in advance