Actually you can have Vbox run on any partition where you have room, all you need to do is point Vbox to where your .VDI file resides.
Hi
caerhaysAs YouCanToo mentioned you can have your VDI's anywhere on your system. I have a folder on one of my backup HDDs to specifically backup my VDI's to. I can then copy them over to another partition whenever I want to re-use them in VBox.
Some VBox users (not you I'm sure

) think that when they first create a VBox HDD and it asks to set a size - say 10GB - it will stay that size, but unless you install loads of apps or keep lots of data on it, in actuality the HDD size is normally far smaller than the 10GB set.
Note: this applies when using the Dynamically Expanding Storage option when setting up the HDD