Came across this issue today on a virtual machine running in a VMware environment. After allocating additional hard disk space to the virtual machine (running Windows Server 2012), the following error appeared when attempting to extend the hard disk;
"There is not enough disk space available on the disk(s) to complete this operation"
Even though the space was showing as "unallocated" in disk management, this error still appeared when trying to extend the volume.
The fix - turns out it was quite simple. From within the Disk Management utility, go to Action > Rescan Disks then try running the extend disk again.