Yosemite migration assistant locks on start, no way to boot anything else
I used Disk Utility from recovery to clone the boot drive as an emergency backup since I no longer had a working time machine backup. When it finished it rebooted half way, glitched the screen and then went back to a normal boot up and I could access the machine as usual. All ok so far.

I suspect that it had something to do with connecting an Ethernet cable to a Yosemite machine while trying to do a migration. I'll not do that again.