Mount drives

If disk is already formatted, You can mount it. You may need to create a file system for it though. 

If you do, Replace XY accordingly, but double check that you are specifying the correct partition

mkfs.ext4 /dev/sdXY

Do that to all the drives/partitions you need to.

Now you should be able to go to "Disk manager" in the GUI or, mount the drive/s manually. 

Terminal:

create a mount point (if you haven't already)

sudo mkdir /directr/to/moun/to

Now mount it:

mount /dev/sbxy /new/mounted/location

Mounting from fstab

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# /this uses the UUID of the drive and mounts it do the directory you want it to go to
UUID=574c96bf-f2cb-49b8-9196-232a24047f94 /mount/dir  ext4    errors=remount-ro 0 0

# /this uses the drive location and mounts it to the directory 
/dev/sdb /mount/dir  ext4    errors=remount-ro 0 0