I have a 2009 Mac Pro (MacPro4,1) which, until recently, had both OS X Yosemite and a Boot Camp install of Windows 10 on it. Shortly after I upgraded to El Capitan, I found that Boot Camp Assistant refused to run, reporting "This Mac does not support Boot Camp".
Furthermore, I am unable to use BootChamp to reboot into Windows, as it reports a cryptic message "Bless failed".
Any ideas what would cause Boot Camp to stop working?