Casper Imaging selects by default (for all hosts without stored imaging data) the first volume named "Macintosh HD" and complains if there is no such volume. @jhenkel shared a nice way to provide Casper Imaging another hint to find its target volume: simply create a file named .jamfTarget in the root folder of a volume and Casper Imaging will select it iff there is no "Macintosh HD" present.
