List VMS at the command line
VBoxManage list vms
This should show something like:-
- "IE11 - Win8.1" {054501a7-7636-407a-b124-9b6c9724ffa4}
- "IE6 - WinXP" {f3fab135-8b15-4b6a-a446-d747854d9688}
- "<inaccessible>" {c7601a5b-eef7-4660-8fdc-a9fbc8dab9d3}
- "IE7 - Vista" {6965d66b-e90f-40f4-92f3-902d5ecc52f8}
- "IE8 - WinXP" {8c1aee87-f77e-48e7-985f-4bdcc6914685}
- "IE8 - Win7" {4ea2240b-a637-4786-a689-f9fd1407f380}
If you have an <inaccessible> on e as shown above, and have been unable to remove it through the VirtualBox app. The commandline can again help you out here.
use the GUID as follows
VBoxmanage unregistervm {c7601a5b-eef7-4660-8fdc-a9fbc8dab9d3}