OS X 10.10 Yosemite was released to the world yesterday, and enthusiastic installing started in M&P this morning. About 30 minutes later that enthusiasm was somewhat tempered by apparent disaster…

It turns out that the Yosemite Installer tries its best to preserve anything which has been installed into /usr/local/ on your Mac by (apparently) archiving it and then restoring after the Yosemite install has completed. But if you have a MacTeX install, with many many small files under /usr/local/texlive/ then it all goes wrong, leading to delays of many hours, or just a terminal hang. This behaviour has been seen on many machines and is a ‘known issue‘ with the MacTeX authors. The workaround is to move the folder /usr/local/ somewhere else, such as your home directory, before the install and then return it afterwards. However there are apparently several issues with MacTeX under Yosemite even after this, so if LaTeX is key to your existence you probably want to follow the updates on the MacOSX-Tex list before migrating.

This problem affects anything installed under /usr/local/ so it’s not just MacTeX but packages like Homebrew which trigger the problem; MacTeX is just the package most people likely have installed.

I have only used Yosemite a little (developer preview) and had a clean install on a new machine, so never saw this sort of problem. Lucky me. I will be waiting for a while to let developers catch up before I migrate my own systems!