To convert a dynamic VirtualBox disk to a Fixed disk, run the following command:
vboxmanage clonemedium disk /path/disk.vdi /path/newdisk.vdi -variant Fixed
using –variant Standard would create a dynamic disk. Then open the virtual machine and remove the existing disk and attach the new disk. Then use the media manager to delete the unused media.
Use the following command to list registered virtual disks:
vboxmanage list hdds