HD does not unmount booted from Ext HD

Normally no problem; recently boots but error notice: "cannot unmount disk; close all applications & files". I re-installed last update to 10.6.3. Trying to find anything that could be open, deleted some apps, etc. So far, nothing works.

Is there a simple way to find any open apps or files I haven't found? I do have Time Machine set on the ext. HD but haven't yet restored from last month to see what changed. Tried booting from 10.6 install dvd. Same answer from Disk Utility.