If you put a faster HDD in a USB 3.0 enclosure you may well get speeds very close to or higher than internal SATA depending on the HDD. Say you put in a 7200 RPM or even a 10000 RPM drive in the enclosure then that may compare well to a 5400 RPM drive on internal SATA. However SATA is the best for SATA drives. Put in a faster drive on internal SATA. Install the games on that. Use the 5400 RPM drive in the external enclosure. 5400 RPM drives do well in an external enclosure due to the lower power requirements and lower temperatures typical to drives of that speed. The right drive needs to be in the right hardware configuration to get the best out of your system.