Clone won't boot

[sdsl]

Originally Posted by gbdoc

I don*t want to risk messing up my external drive which is partitioned for a clone + Time Machine)

Just a random editorial comment -- putting a clone and a Time Machine backup on the same physical drive is not a resilient backup strategy. If a power surge takes out both your Mac and the physical drive, you lose everything. I have seen external drives fail with first one partition won't mount rapidly followed by the other one becoming inaccessible. Much safer to put the clone and the Time Machine backup on separate drives. Then you would feel more secure about installing a clean system on one of those physical drives as you would still have a good backup on a separate device in case something went amiss.