10.6.7 netboot image won't unmount internal hdd in disk utility

I recently upgraded my snow leopard server from 10.6.6 to 10.6.7 in order to use the latest SIU to create some diagnostic images for the new MBP Early 2011. I can create the images just fine and can netboot the early 2011 MBP to the images, but when i try and use disk utility to repair the internal hard drive it says that it can't unmount the drive. Disk utility can unmount all external drives and repair them but not the internal drives.

My source machine is the early 2011 MBP and I placed the 10.6.7 OS on the internal hard drive. Once I made all the adjustments to my source machine, I put the source machine in target disk mode and connected it to my mac mini server via firewire 800. I used the default options for creating a netboot image in SUI. The image is created every time I try this but I can't get disk utility to unmount the internal hard drive for diagnostics. Any ideas?

Thanks,