Can't replace MDM Display Manager with LightDM!

Question

I installed MDM, and found it's incompatible with my VGA hardware. After rebooting I see:

This screen repeats, even if I choose Yes or No

As I have access to the console (by pressing ALT+F2) I tried,

sudo dpkg-reconfigure lightdm

which gave me:

Even if I choose LightDM nothing happens at the next reboot. The first screen comes back!

$ dpkg -l | grep -i mdm
ii mdm                1.0.4-0~webupd8~precise        Gnome Display Mnager
ii mint-mdm-themes    1.0.5-0~webupd8~precise1       Linux Mint MDM Themes

I need LightDM login screen back!

Answer 1

What the message is telling you is that lightdm is already running in the background. So, login to the console-session/terminal by pressing Alt+F2, or choose Exit to console login from the window below.

Now, remove MDM, using the commands below:

sudo apt-get purge mdm      
sudo dpkg -r mdm mint-mdm-themes

Set LightDM as default from this command:

sudo dpkg-reconfigure lightdm

Then reboot. MDM should be gone by now.

LightDM may start, but it's most likely to be in low graphics mode.

To fix this, you have to edit /etc/lightdm/lightdm.conf as following:

[SeatDefaults]
user-session=ubuntu
greeter-session=unity-greeter

As you are unable to edit lightdm.conf through console-session, I may suggest using the Ubuntu Live CD.

(Using the live cd, enter gksu nautilus on terminal, then locate and open /media/yourUBUNTUpartition/etc/lightdm/lightdm.conf from Gedit... then you can do the editing!)

Considering that you haven't installed unity-greeter, download the package for Precise from here, and place it on /media/yourUBUNTUpartition/home/YourName.

That's it! Get out from the live CD and get back on the Console Session. (Alt + F2).

Now install the Unity-Greeter file you downloaded before:

sudo dpkg -i **unity-greeter_0.2.8-0ubuntu1_i386.deb** 

Finally start LightDM,

sudo service start lightdm