OS X unfortunately has quite a loose definition of "free" space: the Finder, Disk Utility and the new Storage pane in About This Mac can all report different values for local, RAID and NAS drives.
Most of the time, the culprit is either a filled Trash folder, or rogue Time Machine snapshots.
If you use a Time Machine disk but haven't connected it for a while, OS X stores its backups on your hard disk as MobileBackups. This can cause your hard disk to fill up pretty quick. It's possible to prevent this from happening by disabling MobileBackups altogether. Read more about here: How to free up space used by Time Machine's local backups
All disks have their own (hidden) Trash folder, and there are probably files left in there taking up space. Try emptying the Trash on your Mac. This empties the Trash on every connected disk, not just Macintosh HD. Conversely if you’re looking for missing footage on a disk, it might be located in the Trash! In that case DON'T EMPTY YOUR TRASH, but check the Trash on your Mac to recover your files.
Still can't find it? Let's make the invisible Trashes folder visible to drag your footage out of it:
Done? Let's hide all that again:
If transfers to Network Drives suddenly fail, with Hedge reporting "Some files couldn't be copied", there's a discrepancy between the reported free space, and actual usage. Some Thunderbolt RAID disks also behave this way, without being an actual network drive.
If you experience these issues, please report the brand and model to us: firstname.lastname@example.org