I have a iMac 27-inch, Mid 2015 the Bluetooth MagicMouse & BT keyboard connect and work perfectly while the OS is loaded. I am able to hold down the option key on the keyboard to get to the boot selection menu and while in the boot selection the KB and mouse are connected. If I choose to boot to anything other than the main OS (ie a bootable USB OS installer with Disk Utilities) the keyboard and mouse are no longer connected or detected. I just get the no mouse / KB connected message and it searches for wireless devices. I am forced to connect a wired KB and mouse to do any setup or repairs.
I have tried turning the mouse and KB on and off. I have tried with several other BT keyboards (both the regular battery models and the new rechargeable models). I have tried resetting the hardware BT module.
I have tried turning the mouse and KB on and off. I have tried with several other BT keyboards (both the regular battery models and the new rechargeable models). I have tried resetting the hardware BT module.