Well normally you would just right click the "Unallocated" space and create a simple volume to do this. Since it didn't work, something might be up. Make sure the connections are in good. After that delete any other partitons you might see on the disk. Multiple partitions could cause the trouble.
If those do not work to get the disk to create a simple volume, then the drive does have problems but it sounds like it is most likely the controller board. The silicon board on the hard drive might have become defective, and those can usually be replaced. You would need to check Ebay usually for a replacement but they are not very expensive typically and it could save the drive. It may not work though.