Without knowing much about how the 500GB drive came to be "unallocated" I would recommend Testdisk if you accidentally deleted a partition. If the drive was formatted then I don't know if Testdisk will help you. There is some critical portion of the hard drive that if corrupted will cause Windows to treat the hard drive as "Unallocated space". I've been successful in recovering one hard drive with this condition using Spinrite. It might be worth a try although the tool is not free and there is no trial version.
