About to purchase a 2017 imac. 3.8 / 8gb ram upgraded to 32 / 512 ssd All files will reside externally with only the system and applications on the main drive. Probably a basic question, but considering cost, is it better to have an ssd external drive connected via usb, or a slower (7200 speed) drive connected through thunderbolt? This is specifically for working on files (such as a large Lightroom catalogue or photoshop files) and not transferring files. TIA.