I tried to install the 10.10.4 update (upgrading from 10.10.2) on my MacBook Pro, and it crashed with the progress bar in the "less than one minute remaining" position. It left the computer unbootable except from my Snow Leopard install CD.
That's odd, usually time machine drives are themselves bootable, to allow you to reinstall the os prior to restoring the backup on them.
I work for the Department of Redundancy Department
