Installing Guest Additions with CentOS CLI only

[asonofmre]

I'm having problems installing guest additions.

I have two CentOS machines installed on virtual box, the first has CentOS installed with the MATE desktop, the second only has the command line.

The first machine I was able to install the guest additions with the GUI no problem, however the second machine is throwing me this error:

Unable to insert the virtual optical disk /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso into the machine server2.

Could not mount the media/drive '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).

Result Code: NS_ERROR_FAILURE (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}
Callee: IMachine {85cd948e-a71f-4289-281e-0ca7ad48cd89}

I'm stuck so any advice would be greatly appreciated