Hey, as already written by somebody in another thread, I can confirm now that I have the same issues on my new iMac with the beta of 10.11.2. The USB ports don't work correctly. If you connect an external drive to USB it will only be recognized on one port and is very slow (same speed as on an USB 2.0 port). My cable mouse seems to work on every port, but the USB receiver for my wireless Logitech keyboard is failing sometimes and I have to pull it out and plug in again. Also there are WiFi issues and sometime after rebooting it seems it does an NVRAM reset by itself because the chime is coming from an internal speaker like when you hold cmd + P + R while booting. I don't know if Thunderbolt is working correctly, because my cable seems to be damaged. Those are even more issues I recognized on the last beta of 10.11.1. The build of 10.11 (15A4310 instead of 15A284) it was delivered with and the final version of 10.11.1 (15B42) seem to work without those issues. Can anyone else confirm this? What about the 4k iMacs, that came with build 15A2301 instead of the normal version? Is it a Skylake only issue?