Hi all, I've a late 2011 27" imac with an aluminium 23" Cinema Display that's been running fine up until I installed 10.9.5. Since then the external display is not coming back on after the screens have gone to sleep. The only way I can get it to work is to pull the power cable from the Cinema Display, plug it back in and when I do that the imac recognises it and it works. I've tried a PRAM and SMC reset but the problem persists, I've even installed Yosemite but I get the same issue. Any ideas? Thanks all.