Bootcamp restart of OS X failures

On several occasions, exiting my Bootcamp Windows 10 partition and attempting to restart in OS X fails hard, required power button hold to shut down my iMac and reboot from scratch. The reboot also fails, requiring a second or third power button hold to shut down again (and again.) This is also typically accompanied by my iMac failing to detect my wired Apple keyboard plugged directly into the computer, as well as my Logitech wireless mouse. A loud constant tone is also generated from my speakers initially, and on the successive reboot failures. Eventually, the computer has recovered into OS X, but no dialog box ever opens to notify Apple of the issue. Does anyone else experience this and have a solution, or is this a software bug that will be fixed in Sierra? Thanks.