USB disconnection when using differential probe
poor CMRR on Diff Probes  
connect 10nF caps from Laptop and DUT to Earth Gnd


If using 2 probes to make Diff probe inductance , the probe inductance is mismatched x% and thus poor CMRR=1/x%


1) Connect scope probe ground clips (earth gnded) to 10nF cap to DUT Gnd
2) twist both probes around each other for better mutual couple A-B cancellation