Ever since I installed 10.12.4 three days ago, I've had constant Bluetooth problems with my Magic Keyboard and Magic Mouse (connected to a 27" mid-2010 iMac): every 1-10 minutes one or both of them lose connection, then shortly after reconnect. Today a new problem has developed, where both drop but don't reconnect, and the Bluetooth icon in the toolbar shows a sawtooth line across it ("Bluetooth Unavailable", according to Apple's documentation). Since I have no working input devices, at that point I need to force-shutdown the iMac by holding down the power button. Likewise, every time I restart my iMac, I see a black progress bar on a grey background, with the Apple logo over it, as if macOS was updating something. 1) I've run Disk Utility's First Aid to check that there are no issues with the disk or files 2) I've removed and re-paired both the keyboard and mouse 3) I've done a Factory Reset (SHIFT-OPTION-click on the Bluetooth icon in the toolbar) for the iMac's BT hardware and both the keyboard and mouse None of this has helped. I'm wondering if anyone else is having BT issues with the latest update, and if you know how to fix it?