So I created this AppleScript to eject all the selected volumes within a second (except my Startup disk, of course!). For each "disk unmount command," the script opens a new Terminal window to eject the volume. This may be annoying, but it's fast. The more elegant way is not to tell Terminal to execute the command, but to do it directly with the Script:
do shell script "disktool -e disk2"
This works, but it takes more than 15 seconds to eject one volume -- and there is no error message if something goes wrong. Servers are unmounted this way, that works very smooth. I don't know why the behavior of the commands differs if executed by Terminal or by System Events. And finally: An image ejected by this script can be remounted easily -- without the help of Disk Utility.

