Converting a virtual disk image: VDI to VMDK to a raw loopback file you can mount

By default, VirtualBox creates virtual disk images in a special format called VDI, which is unique to VirtualBox. Disk images are stored in $HOME/.VirtualBox/HardDisks.

You'll need to convert VDI into another format if you want to run a VirtualBox VM on another virtualization platform, such as VMWare or KVM.

The VMDK virtual disk format is a good choice because even though it originated with VMWare it is supported by other virtualization platforms including VirtualBox and KVM.

VirtualBox enables the conversion using the low-level "VBoxManage clonehd" command:

VBoxManage list hdds # prints a list of disk image UUIDs
VBoxManage clonehd <UUID> -o converted.vmdk format VMDK
cd ~/.VirtualBox/HardDisks/
ls -la converted.vmdk

Once you have converted to VMDK you can use qemu-img, a tool bundled with qemu (KVM's virtualization backend) to further convert VMDK to other formats.

A particularly useful format to convert to is 'raw' which you can then mount as a loopback device:

apt-get install qemu
qemu-img convert -f vmdk converted.vmdk -O raw converted.raw
mount -o loop converted.raw /mnt

Ever had to convert between disk image formats? Post a comment!