This is how you flash to eMMC easily

[nelstomlinson]

Thanks, Zebulon. That's helpful info.

Looking in my system info list of processes while dd is hung up I see a bunch of i/o-looking processes whose status is sleeping or waiting. I suspect that the phone just stops responding after a while. With jumpdrive running there doesn't seem to be any way to interact with the phone except as a disk drive.