If you have USB 3.0 and a fast thumb drive like Sandisk Extreme, Corsair Flash Voyager GT Speed, or Kingston Data Traveler HyperX, it will be faster than a HDD, about like a first generation SSD. USB 2.0 is going to be slower than the HDD, so in this case, an external HDD would be just as fast as and cheaper than a large 2.0 thumb drive
Slower than a modern SSD and faster than a regular HDD.
Well it really matters on your motherboards USB3 controller. USB3 is usually 5GB/s. That is 1GB/s less than SATA.
Look at this website; http://usbflashspeed.com/