First answer by me is that the USB port will be enough power to run an external 2.5" HDD. I have run a power tester on the port and at no time has the power required more than 5v or more than 500ma except for when I power off the box and turn it back on again. It may jump to 670ma for a split second when it first switches on.
Second answer is that if you want to use it for timeshift, a swap file, pause rewind and recordgins you will need to initialise the drive once you have plugged it into the box.
The reason I say about the first answer by me, is that you will have others tell you it will damage your port. 4 months in and mine is going strong. - Also, I don't have my HDD go to sleep, it is always spinning.
So the decision of running it powered by the USB or by external power is up to you as information is conflicting.
