What I found out is that it's a lot easier to use a physical partition or in my case a USB drive as a virtualbox disk image, that way everything gets written directly to the drive.
I have a custom built linux that I did on an external USB drive that I can carry around. It was done in VBox, and if booted in vbox, has built-in guest additions
But I can also grab that same USB drive, stick it in any box, and boot off the USB. Works wonders
And, that way, if I need to get anything off it, without booting, I just mount the partition, and get it.