This may be me but here is the situation
I am communicating via the I2C bus to a Devantech SD21 using an Ardunio Mega 2560
If I use the Arduino on board 5 volt and ground everything works ok.
If I try to use 5 volts to the SD21 and simply use the SDA/SCL from the Ardunio the sketch hangs, the 1st time it attempts to end the transmission.
It acts as if the SD21 is not powered by the external power, I did verify that power does exist at the appropriate pins on the SD21.
Am I missing something obvious ? Have noticed this problem with other sketches as well that use the servo library to fire up an R/C ESC.
Any troubleshooting suggestions