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 prompt you are ready to mount your disk create a mount point, say you want it to be “mydisk”

mkdir /mydisk now mount it

mount /dev/sda1 /mydisk 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)

try writing a file to the disk

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