After successfully extending a partition on a VM, Disk Management shows the correct partition size but Explorer still shows the old size, even after a reboot.

The reason for this is that while the partition has been extended to the new size, the filesystem itself has not. Normally, Disk Management should first extend the partition, then the filesystem, but for some reason, the second part did not happen.

To resolve, perform an extension of the volume using diskpart.

Open an elevated command prompt and start diskpart (type diskpart and press Enter).

List all volumes to find out which volume number corresponds with the drive you need to resize: DISKPART> list volume



Select the volume  ...

On Windows Server Core installations, you don't have Disk Management available. But you can quickly initialize and partition a new disk via diskpart. Here's a quick reference to the necessary commands in order.

1. select disk 1
2. attributes disk clear readonly
3. online disk
4. select disk 1
5. clean
6. convert gpt
7. create partition primary
8. format quick fs=ntfs label="disk label" unit=64k
9. assign letter="E"