After installing Leopard and restarting, Migration Assistant offers to transfer your data:

If I chose “from another volume on this Mac” (connected via USB) or “from another Mac” (connected via Firewire disk mode), I’d immediately get one of these errors:
- “Mac OS X was not found on the volume you selected.”
- “There are no version of Mac OS X on your old Mac.”
- “You can only transfer information from a Mac that has OS X installed.”
The error comes right away, before I actually selected anything. The drive booted without any problems either.
So here is the very non-intuitve fix in case any one else runs into the same problem:
- When the error comes up, just let it sit there for a while (a good 10 minutes in my case).
- You’ll then start to notice that the information from your drive/machine starts showing up (Account info, size of the Applications folder, etc)
- After all the information is populated, click Cancel in the dialog box which will take you back to the previous screen.
- Select “from another volume on this Mac” or “from another Mac” again and this time there will be no error.