Virtualbox adding extra drive to your vm when your storage is getting less
If your virtual vm storage is getting less and you need extra storage space, you need to the the following :-
a) power down your virtual machine
b) add a new virtual disk (vdi) and store it at a specific folder in your host machine
c) power on your virtual machine.
d) goto disk management and you will see a prompt asking you to acknowledge the new disk partition
e) assigned a drive to it and proceed format that drive.
That's all you need to do to add a little extra space to your virtualbox.