Within the last few days, I updated my Linux Mint Debian Edition virtual machine installation to Update 4. Between not following the instructions so closely and problems with the update server, a re-installation preceded the update itself. When all was done, no desktop environment appeared and issuing the startx command revealed that it was one of the VirtualBox drivers that was the cause of the problem. With my being unable to see any files on the VirtualBox virtual CD, something else needed doing and the executing following command (replacing [VboxAddonsFolder] with VBoxGuestAdditions-4.1.12 in my case but it is different for each VirtualBox version) resolved the situation:
When it was complete, a scrambled desktop began to appear so a reboot was in order to set things to rights. Then, I could set to looking at what Update 4 had brought to Linux Mint Debian Edition.