I've found that unplugging my USB drives increases the speed of Explorer* in my Windows system. My system is much more responsive with the drives unplugged--and it only takes a second or two to pick it back up if I need to plug it in again.

*Explorer = the program Windows runs to access contents of locations, show the desktop, create "file save" lists, search for files, etc. I'm NOT talking about Internet Explorer, the Windows browser.