The Raspberry Pi is a nice $35 computer, but it has a maximum of 32GB of storage on the SD card (in 2017, I thought that was the case. It is now 2019. There are 1TB SDXC microSD cards now). If you plug in a large hard drive, such as my Silicon Power 1TB Rugged Armor A60 Military-grade Shockproof/Water-Resistant USB 3.0 2.5" External Hard Drive for PC, Mac, Xbox One, Xbox 360, PS4, PS4 Pro and PS4 Slim, Black from Amazon, it doesn't work!
There's a way to increase the maximum power out of the USB ports, but don't forget to use a good power supply.
It's simple. Type this into your terminal:
sudo echo 'max_usb_current=1' >> /boot/config.txt
And then reboot your Pi.
Done! Your Raspberry Pi should power whatever you plug into the USB port.