When resizing a VirtualBox image you may face the error “Resize hard disk operation for this format is not implemented yet!”.
You can fix this by doing a convert to the VDI disk format before trying to resize it.
It could be necessary to boot the converted image before doing a resize!
Convert format of virtual hard disk:
VBoxManage clonehd disk.vmdk disk.vdi --format VDI
Resize virtual hard disk:
VBoxManage modifyhd disk.vdi --resize [SIZE_IN_MB]
You may also want to resize your partitions.
This can be easily done by using the GParted live cd
That was simple and very helpful, Thnx.