Huge inconsistency in disk space measurement, missing 50% of the disk
You may be seeing the difference between used bytes versus used blocks on the disk.

If a tool iterates over the files on your disk and totals the file sizes in bytes, the total value will not match the used disk space.

Try using other disk tools to determine the remaining disk space. The df tool should provide some insight df -h.

Consider using a tool like GrandPerspective to view the contents of your drive; it appears better maintained than Disk Inventory X. Be aware that these tools show file size and not blocks used.