Windows 10 Update Anniversary I have installed Windows 10 on my Mac with BootCamp. This week, Anniversary Update has been released. I wanted to install it. I have launch wizard to upgrade Windows.

Windows restarts.

Now, even if I press 'Alt' key, the boot menu does not show. Mac OS X starts, slowly... very slowly... very very slowly... When login screen shows up, I can not use keyboard or trackpad.

To start my computer, I need to plug external keyboard and external mouse... Mac OS X is very slow (when I mouse the mouse, I need to wait few minutes before see the cursor movement)

Mac OS X started and logged in, I can use Mac correctly. 

I can access to BOOTCAMP partition in Finder.
In Boot Camp Assistant, I can not see Windows Partition, whereas in Disk Utility, I can...



How can I start again on Windows without having to reinstall everything?
From: Boot Camp: Set the default operating system


  After installing Windows, you can set the default operating system.
  
  
  Set the default operating system in OS X
  
  
  In OS X, choose Apple menu > System Preferences, then click Startup Disk.
  Select the startup disk with the default operating system you want to use.
  If you want to start up using the default operating system now, click Restart.
  
  
  Set the default operating system in Windows
  
  You can’t change your startup volume to an external FireWire or USB
  drive while you're using Windows.
  
  
  In Windows 7, click the Boot Camp icon in the system tray, then choose Boot Camp Control Panel.
  In Windows 8, move the cursor to the upper-right or lower-right corner of the screen, then click Search. Search for “Boot Camp,” click
  Settings, then click Boot Camp.
  If a User Account Control dialog appears, click Yes.
  Select the startup disk with the default operating system you want to use.
  If you want to start up using the default operating system now, click
  Restart. Otherwise, click OK.
  
  
  Restart in OS X using the Boot Camp icon in the system tray
  
  In Windows, click the Boot Camp icon in the system tray, then choose
  Restart in OS X. This also sets the default operating system to OS X.
  
  If you don’t see the Boot Camp icon in the system tray, click the
  triangle to show hidden icons.