Use a utility such as OmniDiskSweeper 1.7.2 to find what files are taking up so much space. If you happen to use Time Machine then you may have a lot of Time Machine snapshots on the drive. You might try the following:
Open the Terminal in the Utilities folder and enter or paste the appropriate command line. Press RETURN and enter your admin password when prompted. It will not be echoed.
To turn them ON: sudo tmutil enablelocal
To turn them OFF: sudo tmutil disablelocal
Note that turning them OFF will also delete all existing snapshots.