Use a symbolic link. You can create one in terminal using the 'ln' command:
Type 'cd ' (notice that space at the end) and then drag and drop you website root folder from the finder into the terminal window then press return.
Type 'ln -s newImageFolder ' then drag and drop where your new image folder is on the other hard drive. Press return.
That should have created a symbol link called 'newImageFolder' and when you open that in the finder, it should go over to the new image folder if itis working correctly.