inside of your virtual machine
1) pick the defragment tool of your choice and configure it to pack data as close together as possible - leaving large areas of free space
(I used http://www.oo-software.com/free/en O&O Defrag Free Edition - a very potent free defrag utility
)2) zero the free space with http://technet.microsoft.com/en-us/sysinternals/bb897443 sdelete
sdelete -c C:\ (on C:\)
or whatever other drive
outside of the virtual machine - on your host
3) VBoxManage modifyhd Disk.vdi --compact
disk worked like a charm and gained me 9 GiB worth of additional harddrive space on my /home partition - and that was only one of those harddisks
Thanks !
