Raspberry Pi B+ USB Power

The Raspberry Pi B+ and the Raspberry Pi 2 B+ by default is not configured to power a 2.5 inch self powering USB hard disk. By default the USB ports on the B+ and 2 B+ only allow USB power for about 400mA. A USB hard disks uses 600mA at a minimum.

To increase the USB power to 1.2 amps

vi /boot/config.txt

add the line

max_usb_current=1

I believe this changes the gpio pin 38 value from 0 to 1 which opens a second resistor in parallel allowing for higher amperage over USB.

NOTE: A Raspberry Pi 3 uses 1.2 amps by default on the USB ports and does not require this setting.