How To Shrink Your Virtualbox VM And Free Up Space For Your Hard Disk

1. Run defrag in the guest (Windows only)

2. Nullify free space:

   With a Linux Guest run this:

   dd if=/dev/zero of=/var/tmp/bigemptyfile bs=4096k ; rm /var/tmp/bigemptyfile
   

   Or:

   telinit 1
   mount -o remount,ro /dev/sda1
   zerofree -v /dev/sda1
   

   With a Windows Guest, download SDelete from Sysinternals and run this:

   sdelete.exe c: -z
   

   (replace C: with the drive letter of the VDI)

3. Shutdown the guest VM

4. Now run VBoxManage's modifymedium command with the --compact option:

   With a Linux Host run this:

   vboxmanage modifymedium --compact /path/to/thedisk.vdi
   

   With a Windows Host run this:

   VBoxManage.exe modifymedium --compact c:\path\to\thedisk.vdi
   

   With a Mac Host run this:

   VBoxManage modifymedium --compact /path/to/thedisk.vdi
   

This reduces the vdi size.