Had problem with some app/program on my 2009 late iMac27 for a while. Finally decided to do a wipe out and install. Have time machine backups from latest rMBP15 2013. And here is what I did wrong and just to share with folks so that you don't have to waste a whole night on this. Step 1: use snow leopard OSX install CD to erase hard drive ( No problem) Step 2: clean install of snow leopard, back to OSX 10.6.2 (No Problem) Step 3: use setup assistant to transfer from my rMBP15 2013 backup (Wrong Wrong Wrong) and here is what happened: after restore/transfer, could not log in with my password (seems a common bug), fixed by launch with installation CD and reset password Step 4: log in, most of the apps programs are grayed out/not usable. Figured out that the backup is from Mavericks and while after time machine transfer the system on iMac is still snow leopard, compatibility issue Step 5: update snow leopard so that I have app store app Step 6: here is another unknown bug, after the update, the system restarts, and I can't log in again, but in a different way. The system recognize my password, but sec after I logged in, it went back to initial "log in" dialogue. No idea why Step 7: Changed password again using the installation CD, and was able to log successfully back into snow leopard with App store Step 8: go to app store, download mavericks (5.3G), took hours Step 9: Mavericks cannot be installed on the computer. Did some research, it is likely caused by my time machine transfer, which was from the late 2013 rMBP mavericks and is different from the one on the App store. So the system won't allow me to install/upgrade to mavericks on the iMac even though the current system is stuck in snow leopard. So I plan to repeat the clean installation of snow leopard again tonight, and will update snow leopard first, then upgrade to mavericks and later use migration assistant instead of setup assistant to restore from rMBP time machine backup. Just want to make sure before I spend another whole night on installing OSX. Is this the correct way to do? Thanks!