I faced this black screen problem right after the latest 4.2.12 update too. This is the solution that I have checked and verified on 2 vm's and it's working. Just follow the instructions and you'll be fine. The problem is caused by an incompatibility of the new VB additions with the xorg subsystem used in 12.04 and previous releases. Luckily, Canonical has released two packages you can install on your guest systems and update them to use the new xorg subsystem and kernel used in version 12.10 without stopping using 12.04. So, let's go and do it:
BEFORE YOU START DOING ANYTHING, I would strongly recommend you clone your affected vm using vbox's clone feature and work on the clone so that you won't damage your initial vm in case something goes terribly wrong.
UPDATE: After some additional tests, I found out that on vm's that are installed on encrypted partitions or use encrypted home folders, the procedure may fail. In order to avoid this, please follow the instuctions on 1, 2, 3 and then PS 1 since that installation is done after the user has logged in their account and everything is decrypted.
1. Start your guest but hold down the shift button right after the vm startup white screen appears, so that you can interrupt normal boot and view grub options.
2. Select recovery mode and let it load.
3. On the recovery mode's blue screen, enable networking so that you can install packages. That will remount your system as read/write. If this fails for some reason, you must follow a slightly different procedure explained in PS 1 but normally you should be ok.
4. After networking has been enabled successfully, select the option to start a root session.
5. In root mode, type cd /opt/VBoxGuestAdditions-4.2.12.
6. Type ./uninstall.sh. This will uninstall 4.2.12 additions from your guest. You may safely remove the VBoxGuestAdditions-4.2.12 folder too afterwards but it's not necessary.
7. type apt-get update to make sure the guest is updated.
8. type apt-get install linux-generic-lts-quantal xserver-xorg-lts-quantal to install both the new kernel and the new xorg subsystem.
9. After the previous installation is finished successfully, from the "Devices" menu, click "Install Guest Additions". Nothing will happen after that at this point, so don't expect any automatic mounting for example.
10. type mkdir /media/isos and mount /dev/sr0 /media/isos. (sr0 <-- zero) This will mount the guest additions ISO in /media/isos.
11. type cd /media/isos to switch to the vbox additions mount and type sh ./VBoxLinuxAdditions.run to re-install 4.2.12 additions. I do assume that since those additions were installed before this procedure, DKMS is already installed on your guest. If not, please install DKMS first (apt-get install dkms) and then proceed with the additions.
The only remaining thing to do now is to either type reboot to restart your vm or poweroff to close it and re-run it. After you issue either command, the blue recovery screen will appear again but do nothing. Wait for the vm to complete whatever it's doing and restart.
The vm will start, showing you the grub menu after that. Select the first option to start your vm with the new kernel and if everything went fine, it will run perfectly ok and the next time you start it, it will start as it used to.
PS 1
If enable networking failed and the vm is stuck doing nothing, power off the vm and restart it going back to recovery mode.
b1 Start a root session and type mount -o remount,rw / to remount your root partition in read/write mode.
b2 Follow the instructions in No 5 and 6 to uninstall vbox additions.
b3 After they have been removed, type reboot and wait for the vm to reboot.
Now that the additions are gone, the vm will start normally but with a low resolution and you will be able to install the necessary packages using a root terminal. Follow instructions No 7 and 8 for that.
If guest additions are to be installed for the very first time, I always prefer to do that with the vm running in recovery mode in a root session or with xorg stopped. This prevents xorg from crashing after this first installation is complete. VBox additions updates do not cause similar crashes though. You can always try to install them in the same root terminal and see what happens and if it fails, you do it the other way around.
If you need to install the additions in recovery mode, you must reboot your vm after the package installation and go to recovery mode of course and follow the instructions on
b1, 9, 10, 11.
I know it looks quite complicated but it has to be done only once.
Good luck!
