1. Right clicked the external hdd and went to sharing tab. At the bottom it said that I had password protection on, so I followed the instructions there to disable it.
2. The 'Share' button in the sharing tab was greyed out so I had to go into advanced sharing and check 'Share this folder'.
3. I then went into the security tab of the properties pane and gave my user account all permissions. My user account is the only account and therefore has admin rights, so I don't know why I had to do this.
4. I then created a new username called 'everyone', and gave it all permissions.
5. Clicked okay and close, and I could then access it via ES on my prime.
The reason I didn't mention the hdd in the OP is because at that point I couldn't access the laptop at all. Once I did the above everything worked. I think having password protection on was preventing me from getting into my laptop at all, as I don't have any passwords in windows.
Thanks for the help! I now have access to all my media on my prime. Good times.
EDIT. Turns out the username 'everyone' doesn't need all permissions. I didn't like the idea of 'everyone' having full control over my hdd so I changed the permissions to just allow reading and writing, no modifying etc. Still works.