Yosemite upgrade fails at late stage, unable to get out of re-try loop
This was a problem solved by simple patience.

I left the machine with "2 minutes remaining" on overnight. After some period of time (presumably greater than 2 hours), the Yosemite install got over whatever was blocking it and completed.

I do not know what the fault was. I am assuming some simple but non-critical I/O was timing out, perhaps repeatedly in a re-try loop. Whatever it was, the predicted time was not updated to reflect the problem, making it look like the upgrade had hung.

That does make this a rather uninteresting fault in terms of Q&amp;A, but I am leaving this here in case anyone else ends up with a similar long-running upgrade and wonders how long to wait.

According to comment by klanomath, many developers (with a large number of project files/ruby/etc) experienced the same problem upgrading to Yosemite. I have just shy of 100,000 files in dev projects in my home folder.



It might be interesting to know if there is a way to back out of incomplete installations (without a disk wipe and re-install) at the late stage my machine was at. My original question is essentially about that. However, I won't be in a position to try or verify any answers.