Nothing different from recovering missing space on an IDE hard disk. Find and delete the files that use that space. The usual tools:
- chkdsk for statistics
- treesize for folder sizes
- cacls or icacls command to give you access to the System Volume Information folder

It's important to do all calculations in bytes (or sectors). Different definitions of Gigabyte differ 7%. And 7% from 500 GB. And 7% of 500 GB is 35 GB. That's just the difference between 500 and 465. That might be the simple answer to your issue.
Definition #1: bytes
Definition #2: 2^30 bytes. That's 1.073.741.824 bytes.