Shrink VirtualBox disk image

Cobra

prerequisite: the virtual disk image is of type "dynamically expanding"
delete and install in the guest as much as you want
defrag your virtual disk, if necessary (Windows guests)
with the guest running, fill the free space on your virtual disk with zeros.
- For Windows guests, use sdelete:
```
sdelete -c c:
```
- For Linux and BSD guests:
```
cat /dev/zero > zero.fill;sync;sleep 1;sync;rm -f zero.fill
```
finally, with the guest shut down, issue the following command in your host system:
```
VBoxManage modifyvdi virtualdiskname.vdi compact
```

YMMV, but I managed to save 2 GB in this way for a Windows XP guest.

Update: in VirtualBox 3.* and later, use the following command:

VBoxManage modifyhd --compact virtualdiskname.vdi

Furthermore, in more recent versions of sdelete, the command-line options 'c' and 'z' has been exchanged. To zero a disk, you now have to use

sdelete -z c: