[MEMORY] Why Can't Access 746Gb On Hard Drive After Partitioning

I just partitioned my external 3.0Tb hard drive using the device manager in Win 7. At first I had one partition of around 2.8Tb, I shrunk the existing partition to around 1.9Tb and then Windows created 3 partitions instead of 2. I ended up with 1 partition of 1.9Tb, 1 partition unallocated of around 100Gb and a 3rd partition of 746Gb unallocated. I was then able to create a simple volume on the 100Gb partition which left me with the 746Gb partition unallocated. When I try to format the 746Gb partition all the options are greyed out - I cannot create a simple volume or delete the partition. I also cannot extend my other 2 partitions to use the unallocated space. When I delete the 100Gb partition I can then extend the 1.9Tb partition and end up with a 2Tb partition and the remaining 746Gb unallocated. Is there any way to recover the 746Gb of unallocated space? Seems a waste to have a 3Tb drive that only has 2Tb of useable space.