Downgrade Lion to Snow Leopard on new iMacs

Got a new iMac? Came pre-installed with Lion? Want to go back to Snow Leopard for whatever reason? (Unlike the Fanboy, I realize we have different needs). I've been trying to migrate everything off of my work laptop (MacBook Pro) running SL to my new iMac.

First I tried just sticking in a SL install disk. It causes a pernel kanic (it just calls it a "panic") and the gray screen if death. Complains of incompatible hardware. Then, I attached my SL Time Machine backup, restarted the Lion-iMac, holding down the ALT key to give me a choice of starting up from different sources (fyi - you CAN see the SL install DVD this way, but when you select it, you get the panic again). Then, I selected the "Recovery HD" option, and tried to install from Time Machine Backup. But again, Lion told me I could not do nada.

Here's what worked for me. Remember, you take full responsibility for trying these tricks. Keep good backups and backups of your backups in case things get screwy.

It requires you to have :

1) Fully upgraded SL to the latest 10.6.8, with all updates in place on your old SL Mac. At some point, in anticipation of Lion, SL had got a Migration Assistant upgrade..

2) A Time Machine backup from your fully upgraded SL Mac. I have mine on a WD external HD. All the cables to attach the external HD to your old SL Mac.

3) A firewire 800 cable (the rectangular ones that cost $45[wtf?!]) - of course, make sure both the computers have the firewire ports. Also, if your TM HD also uses firewire, make sure you have multiple firewire ports on your old SL Mac

Then follow these simple steps:

(NB: 1&2 are probably not necessary, but better safe than sorry)

1) Run Disk Utilities on your old SL Mac, repair disk permissions.

2) Do a Time Machine backup. Turn off Time Machine backups, so it doesn't do freaky stuff during your transfer

3) Mount the external HD with the TM backup

4) Restart the old SL Mac, holding down the ALT key. You should see a gray screen with your HD(s) and "Recovery HD". Double click the "Restore HD"

5) On the Lion-iMac, go to System Preferences, click on Startup Disk, and click Target Disk Mode... to restart the computer in TDM. Attach a firewire cable to the Lion-iMac

6) When the Lion-iMac restarts, you should see the thunderbolt+firewire symbols floating around the screen.

7) Attach the other end of the firewire to the old SL Mac.

8) On the old SL Mac, go to (Disk?) Utilities and click on Restore from Time Machine Backup.

9) Select the TM backup disk as your source, and the Lion-iMac HD mounted onto your SL Mac as the target.

10) You'll get a warning that the target will be erased. Sure. Get it running.

11) At this point, it looked like my old SL-Mac restarted. I unmounted the TM backup disk; couldn't see the unmount button for the (ex)Lion-iMac, so turned it off, got the device removal warning. Unplugged all firewire etc cables. Restarted both computers...

Everything seems good so far. Mail on the new (now SL) iMac rebuilt databases when it first started, but seems just like the older one. Programs run. No crashes yet.

hth