The message you are seeing has nothing to do with your external 24 vdc supply; it has to do with the backplane current loading (24vdc/5vdc) of the ML1200 base unit power supply. I just tried to get the same message by selecting an AWA, BWA & BXB base unit, with (1) 1762-IQ16 and (3) 1762-OB16 (I also changed these to 1762-OW16). The only way I can get that message is if I do not select an AWA, BWA or BXB base and select "Discrete Base" in the I/O config advanced tab. What do you have selected in your I/O config for your base, and which expansion modules are you using?