Compacting a disk image

overwrite empty blocks on the disk with zeroes. See below for applications.
run the VBoxManage tool like this:¹ VBoxManage modifyhd <NameOfVDIorVMHD> compact

Applications for overwriting empty blocks

Eraser (Windows)

[ Homepage ] Install as normal. Then go to Edit → Preferences → Erasing… and select the second Tab Unused disk space. Create a new method with one pass and a pattern of only zeroes. Now go to the “On Demand”-Tasks and create a new one with your local harddisks as target. Run it to perform erasing.

SDelete (Windows, SysInternals)

[ Homepage ] Download and unpack. Run it like:

sdelete -c C:\

dd (Linux)

Use something like:

dd if=/dev/zero of=/junk
sync
rm /junk

This will create a large file with only zeroes and fill up the empty space on your hd.

zerofree (Linux)

[ Homepage ] Install the zerofree package. Run it as the man page says.

Seems like zerofree only supports unmounted filesystems. For this to work you should boot into a recovery Linux inside your VM to clean the disk image.

sfill (Linux)

Install the secure-delete package. Run something like:

sfill -z -l -l /

Transparency-Bug in VBox 3.0

If you just installed VirtualBox 3.0 and noticed that you only see parts of Windows’ titlebars and the icons but no text or backgrounds, try the following:

Run VirtualBox like this: Found here

env XLIB_SKIP_ARGB_VISUALS=1 VirtualBox

1
Older versions of VirtualBox used modifyvdi as first parameter. Also this feature is disabled in VBox 2.1.

VirtualBox