Last year wrote a blog post How To Expand and Extend To Increase Capacity On A Virtual Hard Disk The Easy and Fast Way–Solve the problem of “Out of Disk Space on Drive C” Forever Please refer to this blog post for a step-by-step video of the process.
I ran out of space on my Boot to VHD drive and found that the instructions were identical. Here is what I did:
1) Boot into a different OS using dual boot
2) Open an Elevated Command Prompt:
To run any application with elevated privileges (as Administrator) simply right-click on that program from the menu and select Run as Administrator. An example of this would be if you wanted to run a Command window with elevated permissions just click Start then type cmd then Right-Click on Command icon and select Run as Administrator. This is also known as Administrative Permissions. This tip will work on just about any other program too 
3) The following are the commands I ran:
diskpart
Select vdisk file=”D:Boot.Win8-RTM-MS-VLWin8-RTM-MS-VL.vhd”
list vdisk
expand vdisk maximum=50000
attach vdisk
list disk
– If disk is not online… Use “Online Disk” to bring it online
list volume
select volume 8
extend
list volume
detach vdisk
exit
3) Now I just reboot back into my Boot to VHD drive and it has an extra 10gb of available space
