Environment: VMware ESXi 5.5.x, Windows 2003 VM, Windows 2012 R2 VM.

I needed to expand the system disk of a Windows 2003 VMware virtual machine. I was able to easily extend the disk using vSphere, and Windows displayed the new size. However, I was unable to extend the filesystem using DISKPART. I would receive the following message, when I tried:
“Diskpart failed to extend the volume. Please make sure the volume is valid for extending.”

To get around this, I shutdown my Windows 2003 server. Then added the virtual disk to a Windows 2012 R2 VM. I opened up the Disk Management console via Computer Management. Made the newly added disk Online by right mousing clicking on the disk name (on the left) and selecting Online. Then, I right mouse clicked on the logical disk, in my case the C: drive, and selected Expand. Then, I took the disk offline, but right mouse clicking on the disk name (on the left again) and selecting Offline. Next, I removed the disk from my Windows 2012 R2 VM WITHOUT deleting the file from disk.

Then, all I to do was boot my Windows 2003 VM, and let the chkdsk do its thing.