====== Impossible to open multiple drawings at the same time (SDI variable) ======
===== Symptoms =====
AutoCAD closes any opened DWG when opening another one.

===== Problem =====
SDI (Single drawing compatibility) system variable turns of multidocument interface.

===== Resolution =====
Change value of SDI by typing "_SDI" and set value to 0 (turn on multidocument interface), or 1 (turn off multidocument interface)