If you encrypted the drive when those files were on the drive, they would have been erased. To free up all space on the drive, go into Computer Management (right click My Computer, Manage) and select Disk Management. Find the drive in the bottom right window pane, delete the partition, and create a new one.
The drive will not show up as exactly 320 GB because computers treat a gigabyte as 1024 megabytes, and hard drive manufacturers define a gigabyte as 1000 MB or 1,000,000,000 bytes, so your computer will show a smaller capacity for the drive than it says on the label.