Changing root permissions to user permissions in Ubuntu?

I'm using a USB drive as my main Ubuntu 10.10 installation. I have a HDD in my computer that I would like to use as "external" storage. I've successfully mounted the HDD as /home/user/HDD, but the permissions on the drive say root. Needless to say, I can't write to the HDD. How do I change the permissions to allow writing to the HDD? Thanks in advance.