I'm trying to decide whether to replace an existing hard drive on an iMac 10,1 27inch. I'd much prefer to replace this drive with an external drive as I can get a tremendously bigger drive and the portability of having the OS external. What I don't understand is that I keep reading that USB 2.0 is very slow in reading and writing files, but none of the articles I've found specifies whether this applies to the computer USB port, the external drive port, or both. The iMac has 4 USB 2.0 ports, but the external drive uses USB 3.0. I'm assuming that both ports need to be 3.0 but I can't find any verification on this. It doesn't need to be super fast, but I don't want it to be super slow either. So my question is, can I effectively run an iMac booting from an eternal drive when the computer runs USB 2.0 and the external drive runs USB 3.0?