It's possible that the hard drive has been partitioned in a wrong way. To check for other partitions, go to Start > Control panel > System and Security > Administrative tools > Computer management. Wait to load, then click on Disk Management. Locate the hard disk and make sure that the free space is listed as a partition. In both cases, create a new simple partition and format, it then you can use the available space. Alternatively, go to Start, type Disk Cleanup and use the cleanup for the external hard drive.