I was setting up a new Macbook Air (2013) by running Migration Assistant during the initial setup process, copying from another Macbook Air (2011) using a Thunderbolt cable. The older Mac is running Mavericks and the new Air is still running Mountain Lion. Turns out that wouldn't work and Migration Assistant refuse to recognize both Macs. I read through the Apple support article OS X: How to migrate data from another Mac using Mavericks a couple of times and tried putting the older Mac in target disk mode, as well as updating the new Mac – which was still on Mountain Lion – to the latest (minor) OS version and still that didn't work.
In that end, updating the new Mac to Mavericks and then running the old Mac in target disk mode did the trick. The lesson — Make sure both Macs are running the same OS.
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