Strange activity on external hard disk

Here's one of the scenarios. I have the external hard disk plugged in and started an install of XCode. During the XCode install (installing to main hard disk), the external hard disk was crunching away. Upon investigation, there seemed to be no change in the disk space available on the unit. There have been other times in the past when Mac OS there has been high levels of sustained activity, but I never find anything on the disk that I didn't put there. Any ideas?

thanks.