I figured out the problem. Turned out it was the temperature that shut down the mSATA SSD.
After giving up and waiting for response from Intel Customer Support, I unplugged the power cable from the box. Five days later, when I was going to do some tests on it, I powered up the system, and this time it booted into Windows 7.
The difference I noticed was the box was a lot cooler compared to the last time I touched it. Somehow keeping the power plugged in made the box warm.
I increased the minimum duty cycle of the fan, as described on this article:
Intel� NUC — High temperature causes SSD to stop working
So far the system has been holding up. I am checking with Intel Customer Support if I need a thermal pad for the SSD. Also need to figure out why the box stayed warm with the power cable plugged in.
Thank you.