Migration Assistant Failing
I'd have taken both machines up to 10.10.4 first using the combo updater - as it specifically includes fixes for Migration Assistant.  

Then I'd have followed the instructions rather than try it in target disk mode - you need to be running Migration Assistant on both machines, as you must authorise on both. 

I've successfully migrated two machines in the past month, one from 10.10.4 to 10.10.4  ...the other from Windows XP… &amp; if it can do that, it can do anything ;-)