Impossible d'installer l'image CD de Guest Additions sur Virtual Box

Erreur lorsque j'ai cliqué sur Insérer l'image CD de GuestAdditions à partir de "Devices".

Unable to insert the virtual optical disk /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso into the machine VirtualUbuntu.
Would you like to try to force insertion of this disk?
Could not mount the media/drive    '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).

Result Code: 
NS_ERROR_FAILURE (0x80004005)
Component: 
Console
Interface: 
IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}
Callee: 
IMachine {480cf695-2d8d-4256-9c7c-cce4184fa048}

Je peux voir VBoxGuestAdditions dans la liste de VirtualBox.

homeuser@ubuntu:/usr/share/virtualbox$ ls -l
drwxr-xr-x 2 root root 4096 Jan 14 12:46 nls
-rw-r--r-- 1 root root 64647168 Apr 7 2014 VBoxGuestAdditions.iso
-rwxr-xr-x 1 root root 2176 Apr 6 2014 VBox.sh
-rwxr-xr-x 1 root root 4161 Apr 7 2014 VBoxSysInfo.sh

Une ou plusieurs suggestions ?