A couple of weeks ago I upgraded my mid-2011 iMac to Yosemite and until yesterday all was going well. Yesterday I had the need to run Windows and so fired up VMWare Fusion. I’d already upgraded to Fusion 7 as version 6 has Yosemite compatibility issues.

This is what happened when I ran a virtual machine, running Windows 7 and Office 2013:

• Windows took over 10 minutes to boot up (normally takes about 2 minutes using this particular VM). In fact unless I constantly wiggled my mouse, the bootup process came to a grinding halt.

• Once successfully booted, apps such as Word and Excel took about 2 minutes to load (about twice as long as normal)

• On shutting down Windows, a number of Windows Updates were applied which took over 8 hours!

I restarted the Mac using a bootable Mavericks backup that I created just before installing Yosemite and successfully ran the Windows virtual machine.

So off to the VMWare forum I went in search of a solution and this is what I found:

"You’ve bumped into an Apple bug, affecting all hypervisor programs (a program that runs a virtual machine i.e. VMWare Fusion and Parallels) running on Yosemite on mid-2011 iMacs. Apple is aware of the issue."

The fix is to use this Terminal command: sudo nvram boot-args=debug=0x10

• Using Spotlight or your usual Launcher such as Alfred or LaunchBar, run the Terminal
• At the Terminal prompt, type the above command (or copy and paste it from above) and press Enter
• Enter your Mac account password and press Enter
• You should be left at the Terminal prompt
• To check it has worked type the following at the Terminal prompt: sudo nvram boot-args
• The Terminal should return: boot-args debug=0x10
• Close the Terminal window
• Restart the Mac