So, I have a 27" on order and I keep wondering exactly how to get my stuff transferred to it from my current mid-2007 21" iMac. I won't pretend I know much about Thunderbolt but from briefly looking, I'm assuming I can use my FW cords between the two iMacs, right? Or is there a faster alternative to moving everything over?
The new iMacs don't have a built-in FW port, so you'd need a thunderbolt-FW adapter. Unless you need that adapter for something else, don't buy it just for migrating.
If your old iMac supports Gbit ethernet, just connect the two macs with an ordinary ethernet cable. It will be fast enough. Ethernet is much faster than using a Time Machine backup on a USB 2.0 disk.
I migrated my old iMac to my new 2012 model over ethernet. I think it took about 2 hours to move 700 GB or so of stuff. I started it and went to bed.
The migration worked flawlessly; my whole environment came over, down to little details like placement of icons on my desktop. The Safari tabs that were open on the old mac opened on the new one the first time I launched Safari. 3rd-party apps kept their license keys. Etc. etc. etc.
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If you'd like to migrate the old iMac onto the new one you'll want to hook this up before even booting the new iMac for the first time and then follow the steps when you get to the migration assistant in setup. This will allow you to copy your whole user account with settings from the old iMac.
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You don't have to do the migration the first time you boot the new mac. You can set the mac up, then migrate later. This seems to work easiest if you make a TEMPORARY administrator login on the new mac at first using setup assistant. Then you can use migration assistant to move your REAL login later. Once you have your real account the way you want it, you can disable or delete the temporary user.