Ubuntu has the ability to read MAC file systems (hsf) built into the kernel.
In all probability, it's because the permissions are wrong on your external drive.
If there's nothing sensitive on the drive or only you have access anyway, you could try giving the files 777 permissions, which means anyone can access the files.
In order to prove the concept, you could try the permissions on just one file and see if it makes a difference. At the command line:
chmod 777 /your-directory/your-file
Clearly substitute your-directory and your-file for whatever you want to change.
Just be aware, it's a very insecure method