convert disk from fixed type to dynamic:
VBoxManage clonemedium disk "./oldDisk.vhd" "./newDisk.vhd" --variant Standard
resize disk size
VBoxManage modifyhd "newDisk.vhd" --resize 30000
convert disk from fixed type to dynamic:
VBoxManage clonemedium disk "./oldDisk.vhd" "./newDisk.vhd" --variant Standard
resize disk size
VBoxManage modifyhd "newDisk.vhd" --resize 30000