A toutes fin utiles :
Together with the raspberry PI B+ a new config.txt setting was introduced.
when adding this line the USB power manager will change its output current limit (for all 4 USB ports combined) from 600mA to double that, 1200mA.
is the default, and limits the USB current to 600mA.
Make sure your power supply (and microUSB cord) are up to the task of delivering the extra 0.6A current.
Source : http://elinux.org/RPiconfig
PS : Non encore testé.