When I created my vmdk for my work virt in VirtualBox I made the mistaken assumption that by “dynamic” they meant the drive size would expand as necessary.  What they really meant was that the drive would expand as necessary within the limits of the initial drive size setting – which I stupidly set to 20 gig.

Expanding your vmdk for VirtualBox is really a misnomer.  You have to create a new one of the size you want and then clone your old one over to it.  It’s a little time consuming, but not hard.  Disclosure:  My host is running Windows 7 64 and my VM is running Windows 7 32.  Why bother?  It’s a security thing.

1. Shut down your VM (obivously)
2. Within the VirtualBox Manager, open the Settings for your existing VM, then choose Storage.
3. Add a hard disk to the controller.  Make it the same type as your current disk, and make it the size you need.
4. Once it’s created open a command prompt and head to the directory containing vboxmanage.exe – typically in C:\Program Files\Oracle\VirtualBox\
5. Run the following, altering the paths to match your current and new vmdk files: 
   vboxmanage clonehd "C:\Users\USER\VirtualBox VMs\Work\Work-disk1.vmdk" "C:\Users\USER\VirtualBox VMs\Work\Work-disk2.vmdk" --existing

   1	vboxmanage clonehd "C:\Users\USER\VirtualBox VMs\Work\Work-disk1.vmdk" "C:\Users\USER\VirtualBox VMs\Work\Work-disk2.vmdk" --existing

6. Twiddle thumbs.
7. Once that’s complete, reopen the VirtualBox Manager and head back to Settings > Storage.  Add an existing hard disk to your controller, choosing your new drive.  Disconnect your original drive.
8. Fire up your VM, from within it navigate to disk management, and expand away.