I have been working with the WSM software for about two months and this is one I haven't seen yet.
I had a high up person use their computer for over a month with no problems. Friday, she told me her computer wouldn't come on so I told her to restart her machine and no success.
When I finally made it over to check out her machine, it was showing the NO BOOTABLE DRIVE MESSAGE before coming up to the windows start screen. I checked to make sure she was plugged in and all was correct.