If you initialize a 4TB hard disk as MBR, you can only use 2TB space, the remaining space will be shown as Unallocated. If you use a 2TB+ MBR disk, there's another possible reason why Disk Management is unable to extend volume in Windows 10/8/7.
There's no difference to resize NTFS or FAT32 partition.
Solution 1: Run 'Resize/Move Volume' for D drive in NIUBI, simply drag right border towards right to merge this Unallocated space. As you see, Extend Volume is disabled for D drive, although there's contiguous Unallocated space on the right. To show you the truth, I formatted D drive from NTFS to FAT32. In fact, both Shrink and Extend Volume functions support NTFS partition only, FAT32 and any other types of partitions cannot be shrunk or extended.
Other reasons why unable to extend data volume in Windows 10/8/7 If D drive is Logical and you've deleted it in Disk Management, follow the steps to extend C drive:
