Did you get an awser here? having same issue. suddenly the appliance isn not much use.
I am also having this issue, I really do not want to revert back to using Server.app for this.
Please reply!
Here is how I fixed my issue...
1) Within my Netboot.nbi directory, I navigated to my NBImageInfo.plist and looked for <key>SupportsDiskless</key>.
2) Using your favorite editor (TextWrangler.app or nano for me), edit that key's value from <false/> to <true/> and save.
3) My changes look like the below:
<key>SupportsDiskless</key>
<true/>
Now when I launch Casper Imaging, I am able to let auto run/prestage take off without having issues unmounting the target HD (Macintosh HD).
Does this happen to work for any of you?