Unable to Install Guest Additions CD Image on Virtual Box

I assume your host operating system is Mac OS X and you have installed Ubuntu as the guest operating system.

There is no need to mount the guest additions ISO and install from it if your guest OS is Ubuntu.

Instead, open a terminal and enter the following -

sudo apt-get install virtualbox-guest-utils

This will install the guest additions and is more or less equivalent to installing via the ISO.

I ran into this problem recently (Sept 2017) trying to install Guest Additions into Oracle VM VirtualBox Manager version 5.1.28, in a newly-installed VM for Ubuntu version 16.04.3. The host is Win-10. The problem was that the Guest Additions CD image was already mounted. To fix it:

• In the VM Manager under Settings->Storage, under Controller: Right-click on VBoxGuestAdditions, and select “Remove Attachment." There will be a confirmation box: click “remove.”
• Click the “+” icon next to “Controller," to add new optical drive. A dialog box will open. Choose “Leave Empty-> Okay”
• Open the VM and select Devices->Insert Guest Additions. It should install now. Power off the VM (not the VM Manager) and restart the VM. GuestAdditions should now be working.

For me, the solution was discovering that you have to add an Empty optical drive in the virtual machine settings! If you try to "help" by pre-loading the VBoxGuestAdditions.iso, it doesn't work. The shortcut on the Devices menu "Insert Guest Additions CD image ..." wants to find an empty optical drive to use!