.dmg is busy. Skipping for now.

I'm trying to unmount a disk image after a period of time. I've tried shell scripts, Applescripts and Automator workflows. They all work, and have no trouble in ejecting the disk image, so all I need from Hazel is just to run one of these scripts at the appointed time. Hazel matches the right file at the right time, but refuses to run the script and just returns the "Skipping for now" message in the log.

Is there something about mounted disk images that needs special handling? There are no files opened from the disk image.

Hazel 3.0.16
OSX 10.8.5

Hazel won't process a file if it's busy, which it is if it's mounted. Unfortunately, I don't know of a good solution using Hazel for this because of that.