Depends on the external hard drive type. If it's a portable (2.5") hard drive, you should be fine with a Y-cable. If it's a desktop (3.5") hard drive, use an external power supply.
I don't recommend using an external hard drive without using either. The console may not send enough power to the hard drive if not using a Y-cable or external power supply.
Overall however, I recommend using an external power supply if you can.