VHD Partitions and extending C Drive

I have a server 2008 Standard (Not R2) SP2 VM on Hyper-V.

This VM has a 300 GB VHD which, at the time of setting up the VM, is devided into a 30 GB C drive (OS partition) and 270 GB D drive (Data partition).

I want to extend C drive by another 30 GB so what I did is to shut down the VM, extend the VHD by 30 GB and restarted the VM.

I can see these 30 GBs in the "Disk Management" as "unallocated space" but it doesn't let me extend my C drive to this unallocated space no matter what I do.

For the start, "Extend Volume" is grayed out and diskpart.exe didn't help either.

The VHD in the "Disk Management" appears as a Basic disk.

Please help.