title: How to create a differencing (multi-attach) disk in VirtualBox
body: <p>I have a disk image (Ubuntu Server from osboxes.org) that I would like to use as a base image to create a number of VMs with differencing disks.</p>
  
  <p>When I add the disk image to the Virtual Media Manager and try to set the <em>Type</em> to <em>Multi-attach</em>, I get an error message:</p>
  
  <blockquote>
    <p>Cannot change type for medium 'D:\VMs\Ubuntu Server 18.04.2 (64bit).vdi': the media type 'MultiAttach' can only be used on media registered with a machine that was created with VirtualBox 4.0 or later.</p>
  </blockquote>
  
  <p>I tried converting the disk image with <code>VBoxManage clonhd --format VDI Ubuntu Server 18.04.2 (64bit).vdi converted.vdi</code> because I thought the error message was referring to the version of the actual image, but that didn't help.</p>
  
  <p>I'm using VirtualBox 6.0.</p>