windows 7 - Using Virtualbox, why can my guest (Win7) not see the full disk size?

I need to extend my Windows7.vdi. I tried:

$ VBoxManage modifyhd /mnt/data/VirtualBox/Windows7.vdi --resize 50000

but got: Progress state: VBOX_E_NOT_SUPPORTED - I read that this is due to fix file size but I know my file is dynamic. The file currently is 23.6GB in size but the Virtual Media Manager in VirtualBox tells me its “Virtual size” is 488.28GB but in Windows I can’t see that space, under Disk Management I only see my C: drive to have a capacity of 24.90GB. What's the problem here?

Oh yes the remaining space on my host disk is > 65GB, hence that should not be the limit.

Here’s a screenshot of the Disk Management dialog in Windows next to the Virtual Media Manager from VirtualBox:

If you look at the screenshot, no empty space is being made available on my disk. It won’t let me either create a new partition nor does it let me enlarge the existing one for some reason...

Answer

Assuming you use the right number in your command to enlarge the disk:

Increasing the disk does not make the C: partition larger, it just makes the underlying disk larger. Inside Windows, you could now create a D: drive on the extra space, or extend the C: drive to encompass the extra space you gave the disk.