There are two things that result in the space you see on the drive not matching the advertised space. First, the box measures space in GB of 10^9 bytes, while the OS measures space in Gigabytes of 2^30 bytes. The computer community has even made up the awful term gibibytes (GiB) to mean the Gigabytes that your system measures space in. That's 596 GiB right there. Plus, some of the space is taken up in formatting and partitioning overhead. So 591 GiB is about right.
Your subject refers to 320 GB missing - that would be unusual.