Would this be dangerous ?

[DMcCoy]

You sure can; I've had to expand the boot volume with diskpart on a couple of servers this year alone. The only trick is to boot up with WinPE first - just like @SYNACK has already stated.

 

Or if you can't be bothered to build a PE image, a 7/2008 R2 DVD/iso and shift-f10 to get the command prompt from the setup screen.