first we need to identify the disk(s):

  sudo blkid

This will list any recognised devices:

  /dev/mmcblk0p1: LABEL="RECOVERY" UUID="0403-0201" TYPE="vfat" PARTUUID="0006dd3f-01"
  /dev/mmcblk0p5: LABEL="SETTINGS" UUID="705f6e2b-fac6-4f33-8611-d57a9c9f04e1" TYPE="ext4" PARTUUID="0006dd3f-05"
  /dev/mmcblk0p6: SEC_TYPE="msdos" LABEL="boot" UUID="1495-189B" TYPE="vfat" PARTUUID="0006dd3f-06"
  /dev/mmcblk0p7: LABEL="root0" UUID="759bca6b-5766-4941-b830-cdbfcd861107" TYPE="ext4" PARTUUID="0006dd3f-07"
  /dev/mmcblk0p8: LABEL="boot-rbp2" UUID="200C-EA5B" TYPE="vfat" PARTUUID="0006dd3f-08"
  /dev/mmcblk0p9: LABEL="root-rbp2" UUID="26d10fa3-fe0a-4044-b24a-9b85c2079122" TYPE="ext4" PARTUUID="0006dd3f-09"
  /dev/mmcblk0: PTUUID="0006dd3f" PTTYPE="dos"
  /dev/sda: PTUUID="279bf5b4" PTTYPE="dos"

In this example, the first 6 items are the SD card that Raspbian booted from /dev/mmcblk0px. The last device /dev/sda is a USB Hard Disk. This is the disk I want to add to Raspbian.
Now that we know the disk we wish to work on is /dev/sda we can use:

  sudo fdisk /dev/sda

  enter p to display partition information
  
  Disk /dev/sda: 298.1 GiB, 320072933376 bytes, 625142448 sectors
  Units: sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disklabel type: dos
  Disk identifier: 0x279bf5b4

We can see the size is 298.1 GB.

  Use the d command to delete existing partition
  
  No partition is defined yet!
  Could not delete partition 81165   

In this case, there are no partitions to delete
To create a new partition, use:

  n - This creates a new partition
  p - This is for a primary partition
  Enter - To default to partition 1
  Enter - To select first sector
  Enter - To select last sector.

You should now have a new partition.

  p - To display the new partition
  
  Device     Boot Start       End   Sectors   Size Id Type
  /dev/sda1        2048 625142447 625140400 298.1G 83 Linux

The changes need to be written to the partition table:

  w - To commit changes
  
  The partition table has been altered.
  Calling ioctl() to re-read partition table.
  Syncing disks.

Now run the following command to see your disk, which will now include /dev/sda1

  sudo fdisk -l

There will be a large output, but the important part is at the end:

  Device     Boot Start       End   Sectors   Size Id Type
  /dev/sda1        2048 625142447 625140400 298.1G 83 Linux

/dev/sda1 is the partition we have just created on device /dev/sda
Now we need to create the file system:

  sudo mkfs /dev/sda1
  
  mke2fs 1.42.12 (29-Aug-2014)
  /dev/sda1 contains a ntfs file system labelled '300gb'
  Proceed anyway? (y,n)    <---------------------------------You have to say 'Y' Here.
  
  Depending on drive size, this will take a minute or two
  
  Creating filesystem with 78142550 4k blocks and 19537920 inodes
  Filesystem UUID: 6af40af7-759f-4ee5-afea-882e9f58f17e
  Superblock backups stored on blocks:
          32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
          4096000, 7962624, 11239424, 20480000, 23887872, 71663616
  
  Allocating group tables: done
  Writing inode tables: done
  Writing superblocks and filesystem accounting information: done

After the superblocks are created and you get a command prompt, Now you are ready to mount your disk.
Lets create a mount point and call it NewDisk

  sudo mkdir /mydisk    <-- This creates a mount point (a folder) to mount our disk, the folder is called mydisk

To Mount the Disk

  sudo mount /dev/sda1 /NewDisk    <--- bear in mind that your disk might not be sda1

Use df to verify disk is mounted. If you reboot you will need to remount it (you might want to add it to /etc/fstab)

  df
  
  Filesystem     1K-blocks    Used Available Use% Mounted on
  /dev/root        7928236 3577848   3924612  48% /
  devtmpfs          469544       0    469544   0% /dev
  tmpfs             473880       0    473880   0% /dev/shm
  tmpfs             473880    6520    467360   2% /run
  tmpfs               5120       4      5116   1% /run/lock
  tmpfs             473880       0    473880   0% /sys/fs/cgroup
  /dev/mmcblk0p6     64366   20436     43930  32% /boot
  tmpfs              94776       0     94776   0% /run/user/1000
  /dev/sda1      307665360   64344 291972508   1% /NewDisk    <---- here is our new disk

try writing a file to the disk

touch /diskname/test was the file created or did you get an err?