Mark,
Click on the external drive and click on the relevant folder (or the drive if using the whole thing).
Hold down [CMD] and press
to bring up the 'info' box.
Go to the bottom of the info box and find 'Sharing & Permissions:'.
Expand Sharing & Permissions....
Here you can see that this folder's permission is set so that only admin has read & write access.
To change this click on the padlock (bottom right) and enter your password
Click the '+' and add YOU and give yourself read & write access.
If this does not fix the problem let me know.