It is true that in theory, they should be hot-pluggable. But try plugging out a hard-drive while your system is running, bad things happen. USB devices, on the

It is true that in theory, they should be hot-pluggable. But try plugging out a hard-drive while your system is running, bad things happen. USB devices, on the other hand, expect to be hot-plugged, and take appropriate measures for that. For example, removable media flash drives don't do as much caching in volatile memory as internal SATA hard drives. Another example would be a graphics card, which, if you plugin while the system is running, won't get recognized. But if you want to use Thunderbolt to power external displays that carry their own graphics cards, the underlying assumption being that the card already knows how to talk PCI, then you can see that there will be a problem.