I upgraded my Mac last week to Yosemite so that I could download Lightroom 6, which I did and I uploaded 2 photo sessions in LR6, worked on pics for the whole day. Next day Mac says startup disc almost full, now can't open LR6. I deleted some photos and movies which did give me a lot more space and now startup disc almost full window not popping up anymore, but still can't launch LR6 only LR3. keeps showing me the rainbow swirl when LR6 tries to launch. Will show swirl for hours.
I would suggest you to please uninstall and reinstall Lightroom 6 once on the computer.
Please let us know if it helps.
ok. Thanks so much. I will give that a try.