Keep in mind that 2.5W are the idle power requirements. My boards need about 500-600mA (mostly depending on GbE) at 5V idle current. Under load the A64 and on-board peripherals need 1.5-2A ... the USB ports have compliant 500mA integrated current limiters which would also cut off any load spikes. I always use external power supplies for my external HDD/SSD.
Come have a chat in the Pine IRC channel >>