I have a copy of debian running on a busted ThinkPad without an internal monitor.  It would be nice if the command line didn't revert to a 640x480 resolution on the external.  Solution: completely disable the internal monitor, so linux auto-sets the monitor resolution according to the specs of the external monitor. src.

Find the name of your monitors.  My internal card is an intel, so I can look in /sys for the EDID file (which has the EDID name, which is what we want).  src.
find /sys -name edid
Based on the output of that command, the name of my internal display is
With that information, I'm going into GRUB and disabling the display.  Note it will not work at all after this, unless you change the setting back. 
sudo nano /etc/default/grub
edit the line from
(or whatever it was to begin with) to
Keep whatever settings were already there.  Update GRUB, and reboot the computer.  
sudo update-grub