virtualbox.org

The Issue
In 64-bit Ubuntu(and any other x64 Guest OS, i think)... if you try:

• Power Off The Machine
  • it might creates a dialog saying that is powering off, counting seconds left... and stays there forever!
• Install Guest Additions (or mount any ISO file after 64-bit guest already booted up)
  • it might freezes the whole VM app. You will be forced to Force Quit.
• And probably more that I didn't found out.

I had a similar problem in Windows 7 64-bit too.

Related
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Fortunately, I found a very simple fix.

The Fix
Just enable "Use host I/O cache" for every storage in the guest settings. Simple as that. Don't know why it was not enabled by default. But it shouldn't make the Virtual Machine freeze, anyways.
Image:

It seems simple, but enabling that checkbox solved ALL my issues.

Logs
I collected some logs and force quit reports... maybe it's useful if someone could send it to developers.

Thank you so much! I've been trying to find a solution for this ever since I started using VirtualBox, and it was so simple.