[dnanian]

You know, Falk, this is weird. The only thing going on there is a grep for the words "Could not unmount". I can't see how that would be a conflicting matcher.

Do you have an unusual or 3rd party grep installed?

If not, what happens if you run:

echo "foo" | grep -q -F "Could not unmount"

at the terminal? -q says "don't give an error or anything else" (and you're getting an error)... so something strange is wrong.