I am running TechTool Pro 4 from one volume to repair another, but the program says it cannot unmount the volume to repair it. Why not?

If you boot your Macintosh from one volume and cannot unmount another volume, it is because the other volume has open files on it. You can use the "lsof" command to find the open files on a volume that does not unmount or eject. To do this, open Terminal (typically located in /Applications/Utilities) and type:

sudo lsof | grep

Leave a space after the "p" in "grep". Then drag the icon of the volume on which you want to locate the open files to the Terminal window. The pathname for the volume is pasted into the window for you. Press the "return" key. Then, at the prompt, enter your administrator password and press "return".

It may be possible, given the names of the open files that appear in the list, to determine what process has opened them. Finding unexpected open files is frequently a problem with cloned volumes.