Official documentation says:
Typically, the model B uses between 700-1000mA depending on what peripherals are connected; the model A can use as little as 500mA with no peripherals attached. The maximum power the Raspberry Pi can use is 1 Amp. If you need to connect a USB device that will take the power requirements above 1 Amp, then you must connect it to an externally-powered USB hub
That means I can connect any device that uses 5V and current less than 1A.
Why cant I connect my external hard drive that operates at 5V and consumes less than 500mA (this is the maximum current USB 2.0 can provide) ?
I am using Raspberry Pi 2 and Raspbian as OS
NOTE: I am only connecting the power source (that can provide more than 1A) and a Ethernet cable