I just found interesting property of Raspberry Pi.
We know that a USB device can use power from a host machine, aka computers. In other words, a USB mouse or a USB HDD can be powered from a computer.
The host machine, i.e. computer, is supposed to be plugged to a power outlet.
The Raspberry Pi has a dedicated power port. I have connected a power adapter to a wall outlet or a USB power cable to a powered USB hub, etc.
What I found is that…. if the powered USB hub is connected to a Raspberry Pi’s USB port not its power port, Raspberry Pi can GET power from the USB hub and was able to be booted.
Probably it’s because it drains very little electricity and probably people may want to make a device with a RPi as a kind of independent device. For example, let’s say you make an intelligent HDD ( or SAN ) using RPi as its brain and package the HDD and RPi in a same box. Then, the small intelligent HDD can be hooked up to your network while it’s connected to your USB hub or other device which is to propel USB devices.
I didn’t expect such use case, but they prepared the RPi for such occasion by design!