> TB2/mDP can't carry power
> > TB1 should carry some power.
> > I was able to run a thunderbolt capable seagate external 2.5" hard drive
(all this with the caveat of "I am not a physicist, but…")
All data is essentially energy/power, right? Isn't all communication, a change in energy state? Therefore, all data is essentially a transfer of energy/power.
Even the older standards - serial, firewire, USB 1, USB 2, USB 3, etc - carried some power, although (in general), a fairly small amount. I've used external USB-2 drives without a power source, except that provided by the USB-2 cable.
The maximum power-draw of a USB device is documented in the standards, but generally, USB 2 is permitted to draw 500mA, at 5V: 2.5W.
The power of TB2/mDP wasn't limited by the plug, but rather by the specifications of the standard/protocol.
USB-C is intended to provide significantly more power. All USB Type-C cables should support 60W. High-power cables should support 100W.
The older standards can (just about) provide enough power to spin a hard-disk, but the newer standards can power and charge a laptop.