Barebones documentation of my setup process.

Download latest hypriot image (https://blog.hypriot.com/downloads/) & extract from the .zip
Use Etcher to flash the SD card.

The default user is pirate.
We'll assume that your external drive is mounted at /dev/sda1
I've got an external drive that's shared with a windows device, so it's NTFS formatted. Skip that bit if you don't need it..

sudo apt-get update
sudo apt-get install ntfs-3g (Needed for read & write access to NTFS)

Find USB drive with lsblk
view the UUID for your harddrives by entering: sudo blkid <sda1>

Add line to /etc/fstab to mount USB drive to /mnt/
UUID=D8ECAD3CECBC271A /mnt/ExtHDD ntfs-3g uid=1000,gid=1000,defaults,umask=0022 0 0
/dev/sda1 /mnt/ExtHDD ntfs-3g uid=1000,gid=1000,defaults,umask=0022 0 0

(where uid & gid are found from "id pirate" & umask is there to limit permissions). Don't forget to create the mount point (/mnt/ExtHDD in this case) if you don't already have it.

Install Samba
sudo apt-get update
sudo apt-get install samba samba-common-bin
sudo nano /etc/samba/smb.conf
& add at the bottom the bit inside the "---8<---":

Comment = Pi shared folder
Path = /mnt/ExtHDD
Browseable = yes
Writeable = Yes
only guest = no
create mask = 0660
directory mask = 0771
Public = yes

sudo smbpasswd -a pirate
sudo /etc/init.d/samba restart