I have a problem, on 3B, of external USB disk (an SSD) that I use as a root filesystem, randomly disconnecting if I move the Pi around or change its position or orientation on my table. It's located under a display at the moment.
I've been unable to really figure out whether the problem is with the power connector or the USB connector to the disk. I have tried changing cables, but I noticed that the combined power requirement of the Pi and the disk is such that a normal micro-USB cable will not do and I see the lightning symbol all the time. I have two premium micro-USB cables. Neither show the lightning symbol, even if I stress test both CPU and disk. But they are quite stiff. Which made me suspect that the normal, more flexible cable would not cause those disconnects.
The cable from the Pi to the disk is a short one. So it should not cause a voltage drop. But when the disconnect happens, the Pi never reboots. It just kind of "loses" the root disk, and the only way forward is to reset it.
I wonder if any, some or many have experienced something like this. I will keep on shuffling all my cables to see what is it really about.