Why do external drives spin up for webpage rendering? I recently went to some webpage in Chrome and noticed that my external harddrive had to spin up before the page would render.  Does anybody know why OSX does this?  

I've noticed it in a few other programs when I get to a file/open dialog, but rendering in Chrome?
In Activity Monitor.App you can see for each process Open Files and Ports. You can do the same things with Instruments.app which is part of the Xcode/Development bundle. 

You should dig it that direction, it can be that Chrome loads something for your external Volume.