I also tried to use an 5v extenal power supply to power directly the arduino beacause i also suspectd that could not be able to supply enougth current like you sugest. 
Or should be connected the pull up directly to an 5v external ? if so will be no dangerous to got different mixed tensions? From 5v usb and 5v external?
The project works in this way, but i just wonder why i need more pull-up resistors. Because i almost sure i only use one bus.
Yeah will do that when i got to work.
Will put the images and code later to be according to the rules of forum also.