I am running Migration Assistant (over a network) to transfer an old Mac's setup to a new one. The process starts fine, but every time I try it the target computer freezes mid-transfer (at the moment the source computer has shown '9 hours 34 minutes' remaining for the transfer for hours). The target computer's screen is black, and it does not respond to keyboard or mouse input, but I can tell it's on from the fan whir. It does not respond when I press the power button as it would were it asleep or off. I have to hold this down to force it to shut down. I've restarted the whole process several times - what should I do?
Since this has failed more than once, it's likely you'll have to effect some repair or change tactics.
Here are a few that seem likely - you'll have to choose based on your specific situation:
Do note, that one possible cause of the freezing could be hard drive failure. If that's the cause, you want to get the important files off first and avoid 'fixing' anything. (Imagine a potential house on fire - you may not have time for everything, so prioritize and just get things out now - organize them later.)
All the other causes are not as dire and you may have time to try things again and again, just wanted to plant the idea that this might be a warning so you could get professional help if there are valuable or irreplaceable files on the freezing computer.
Transfering over Ethernet and a local area network is always problematic. Use Target Disk Mode over FireWire to directly connect one Mac to the other, and you will probably have much better results.