This happened to me last night with my HD500. Only had the unit a short time. Finally, got it sounding really great, doing some recording with my Windows 7 laptop. 
  
Then, I get error messages, Blue Screen, shutdown. I look down at the Line 6 and it's showing the continuous LINE 6 screen. 
  
After doing the hold down left arrow, plug in, it started up normally. The only thing I don't know is...does this reset the unit to factory settings?  I'll find out later today. 
  
Thanks for the tip! 
  
steve