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.
When I add the disk image to the Virtual Media Manager and try to set the Type to Multi-attach, I get an error message:
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.
I tried converting the disk image with VBoxManage clonhd --format VDI Ubuntu Server 18.04.2 (64bit).vdi converted.vdi because I thought the error message was referring to the version of the actual image, but that didn't help.
I'm using VirtualBox 6.0.
After adding the image to the Virtual Media Manager you first have to create a throwaway VM that uses the image as its disk. Only then you can change the Type to Multi-attach in the Virtual Media Manager. This will un-attach the image from the VM, the VM can then be deleted (choose Delete all files).
From that point on the image is registered as a Multi-attach image and will create a differencing disk whenever it is attached to any VM.