Hi,
Recently, I was building a custom 3d printer using the Smoothieboard and kept running into a random disconnect issue regardless of the software used (Cura, Simplify3d, etc). The reason for the disconnect was surprising and that's the purpose of this post.

I noticed that every so often Smoothie would disconnect from the computer, sometimes immediately, sometimes hours later. I had a Honeywell fan (HT-900 model) by the printer to assist in cooling for vase mode projects. I noticed one day that if I quickly toggled the fan on/off, I could get the Smoothieboard to disconnect, so I immediately thought a shielding issue. I picked up a shorter USB cable and overly shielded it using MuMetal and to my surprise the issue was still there. Then I started thinking maybe the issue is that I have both the fan and the printer/computer all on one circuit and maybe the transient load of the fan turning on/off is causing an issue? A little bit hard to believe since the fan isn't pulling much power, but I especially dismissed it because I had the fan, printer, and computer all plugged into a UPS on a dedicated 20 amp circuit. So you would think, the UPS would help negate any transient power spikes that could affect the printer? However, once I unplugged the fan from the UPS and put it on another outlet (same circuit though), the problem completely went away.

I'm still surprised by the result, but the moral of the story: Be very careful what you have plugged in with the printer, even if you have a UPS.