One of my partitions, of 270 GB appears to be empty, but when I try to copy a folder to it of about 95GB it tells me that there isn't enough room. File — get info says available 91 GB, used 178 GB.
If Derek's sensible suggestion doesn't work, open Disk Utility, select the rogue partition and click on 'Erase'. Beyond that, in Disk Utility, select the Drive that contains your partition and click on 'First Aid'. If there is a problem with partition mapping this should fix it.
Eric Jervis 