I have a new MacBook with no firewire, USB only. I'm looking at a small OWC Mercury external HD, but I'm confused as to why the the faster 7200 RPM with 500 GB is 25% less ($149. vs $199) than the 5400 RPM with 500 GB? What is the difference between 8 and 16 MB of cache? Here's the linky to the table that has the prices: http://eshop.macsales.com/shop/firewire/on-the-go Thanks for your input.