This specific content was written 14 years ago.
Please keep this in mind as it may be outdated and not adhering to best-practices.
I came across a situation where my VMs were taking too much space from my C drive. The instructions below are from the forum post here.
“The most common I found is to remove your VMs from the manager without deleting the files. “Remove the VMs from the manager without deleting the files. You can then move the entire VM folder (not just the bare VDIs) to a new location, before adding them back to the manager.”
Here’s how I did it… Use these instructions at your own risks 
Jan 20 2013
How to move “Virtualbox VMs” files
“The most common I found is to remove your VMs from the manager without deleting the files. “Remove the VMs from the manager without deleting the files. You can then move the entire VM folder (not just the bare VDIs) to a new location, before adding them back to the manager.”
Here’s how I did it… Use these instructions at your own risks
By Menelaos Bakopoulos • Virtual Box 0 • Tags: Virtual Box, VM files