I have a late 2015 5K iMac plugged into a CyberPower 850PFCLCD. My iMac kept freezing during startup- the loading bar would get stuck about half way. After doing some typical trouble shooting (i.e. PRAM and SMC reset) without any success, I started unplugging USB connections to try and isolate the problem. Turns out, when I unplugged the CyberPower UPS USB cord, the startup freezing went away. The usb connection between the iMac and UPS allows me to mange settings in case of a power disruption/outage, so it's a fairly important USB connection. Anybody else experiencing something similar with a UPS? Any other ideas that would allow me to have the UPS plugged in via USB and still allow the iMac to startup properly?