Why: Can boot from Lion install image, but not Snow Leopard Server

It is probably common knowledge that the App Store download of the Lion installer contains a disk image, which can be burned to a dual-layer DVD and used in a drive for doing a clean install on compatible hardware.

I installed a Lion VM from this disk image. Parallels boots from that disk image without incident, after which installation proceeds just as it would, if a DVD burned from that image were used on a physical Mac.

While I was able to install a Snow Leopard Server (SNL) VM from physical media, the procedure above doesn't work. In the SNL case the disk image was generated in Disk Utility from the physical media. When I select that image file as the installation source, the VM first reboots, but then I am greeted by an alert that states:

There is nothing to the effect that it is an unsupported guest OS, which I have proven not to be the case (see above). Rather, this appears to indicate that there are different disk image formats, ones that can be booted by a virtual machine and ones that can't. If this hypothesis is true, it begs the question of how to make a disk image from installation media that will allow being booted from by a Parallels VM.

I have tried making a .cdr (or .iso) disk image form the SNL installation DVD, but the behavior is identical.

Any ideas?

 

I downloaded a 10.6 Server DVD image from the Apple Developer Connection (ADC, membership required). Parallels allowed me to install from that one.

This still doesn't, of course, answer, as to why a disk image generated by Disk Utility doesn't.