Well, this is about 6 months late, but it works.

There are a few Mac models, the iMac and Mac Pro being two, that originally shipped with Mac OS X 10.6.x but after the 10.7 release started shipping with 10.7. Now, since the hardware is the same, it should be possible to run 10.6 on these computers. However, it seems to be a little tricky to do.

You will require a valid Snow Leopard license to do this. As best I can tell, a Lion license DOES NOT let you downgrade for free. Fortunately, Apple still sells Snow Leopard.

The trick is to repartition the target drive (the boot drive in your Mac) using disk utility on a 10.6 computer. The other trick, is that you might need to install via target disk mode, and update the OS.

What you need to do.

Step 0 - optional - back up your Lion install and/or restore partition.

Step 1 - Target disk mode the computer (or if it's a Mac Pro, you can swap the drive into another computer).

Step 2- using Disk Utility on a 10.6 computer repartition the drive. It is not enough to 'erase' the drive. You need to create a new partition. This will delete the 10.7 restore partition, and any other crap apple has hidden scattered through the drive.

Now, depending on what media you have available, you may be able to run the 10.6 installer as normal. If it shipped with 10.6 or .1 or .2, then you can use also the 10.6.3 retail installer. However, if the model originally shipped with 10.6.3 or higher and you don't have the custom 'grey' install disks for that model, you will need to use some trickery.

If you have appropriate installers:

Step 3 - disconnect from target disk mode (or if you've swapped the drive out, swap it back)

Step 4 - run the 10.6 installer as normal.

If you don't have appropriate installer:

Step 3 - using target disk mode connect (or swap the drive over) connect to an older intel Mac for which you DO have an appropriate installer.

Step 4 - boot from the 10.6 installer on the older intel Mac, and select the target disk moded disk (or swapped drive) as the install location.

Step 5 - install 10.6 to the drive. You can run all the way through the installer, or you can just quit it when it reboots to the language screen (command - q).

Step 6 - install the 10.6.8 combo update. If you've quit the installer at the initial set up screen, you can run the 10.6.8 combo updater from any 10.6 computer (using target disk mode or swapping the drives), and simply change the install location.

Step 7 - Reboot the new computer (swap the drive first if needed).

So you've just bought a Mac Mini server with Lion Server preinstalled and noticed that Apple in their infinite wisdom have supplied it with two drives, but has not set them up as a mirror RAID.

If you've a server with two drives, probably mirror RAID is the way you want to go.

Furthermore, in their additional infinite wisdom, Apple have changed Lion so that it doesn't work 'properly' with Mirror RAIDs as a boot volume. To be precise, the restore partition isn't supported when using a Mirror RAID as the boot volume. This creates a second problem.

The old fix, with 10.6 Server, was to simply enable RAID on the boot volume, and then add the spare drive to the mirror. A few minutes work. This won't work anymore because of the restore partition. The way around this is to repartition the drive OR destroy the partition table by creating a RAID. Both these options will kill the existing volume meaning you need to reinstall Mac OS X... but without a recovery partition, and without DVD installers (Apple in their infinite... don't include them), and because Server was preinstalled you are stuck to download from the App Store.

So, time to get devious. The approach I am now taking is pretty straight forward. You will need another computer running Lion. Basically, you image the fresh 10.7 Server, blow away the existing volume, create your mirror, then image back.

Step 1 - image the current drive. I do this by putting the server in target disk mode, hooking up to a 10.7 computer, and use Disk Utility to create an image. This is important. For some reason Disk Utility on 10.6 doesn't seem to work quite right.

Step 2 - Using Disk Utility destroy the current drive set up. If you don't want a RAID, then you just need to repartition the drive, which will destroy the Restore partition. Alternatively, create a mirror RAID which will destroy the existing partitions anyway.

Step 3 - Create a partition on the RAID mirror (if necessary).

Step 4 - Scan the disk image for restore. This will take a few minutes.

Step 5 - restore the disk image to the created PARTITION. This will take a few minutes more.

Step 5a - optionally, you can run a manual volume verify on the restored image.

Step 6 - Boot from the new drive and verify working.

Now you have Lion Server working and booting from a RAID mirror. And although you don't have a restore partition, you will have an image of the server which you can use to restore in just a few minutes.