log in as root and use chown and chmod
root@yourBox:/# chown -R user:group /hardDrive/dataFolder
root@yourBox:/# chmod -R 755 /hardDrive/dataFolder
where 'user:group' can be just your username (say, joe), or your username and a particular group (joe:users). The second is recommended so that you don't run into permissions issues with that data for other normal users.
The chmod will change the actual permissions so that even globally users can read and execute the data/programs, but only the owner can modify (that's what the 755 means). For more info on both of these you can find them in the man pages, should you dare :-p.
(EDIT) I forgot to mention that you should change those commands so that they suit the top level folder of wherever your particular hard drive is located. for instance, if your extra hard drive gets mounted at '/hardDisk2', that would be what you put at the end of both commands. Just wanted to be extra clear :-D. (/EDIT)