re: I have removed the motherboard and a selection of other components (PSU, CPU + Cooler, case fan, thermal sensor, Power -On button, RAM, etc) including the IO-USB assembly (part no 5F85N ). if you have included everything stock including oem power button module and moved system to a test bench, it should work without issue and error.  Indeed it is a good sign that you are not getting IO cable error based on your report.  System should not care whether every stock component is assembled correctly in a Dell case vs non-Dell case. Re:  both USB 2 ports not active nor detect a working USB memory stick when inserted both USB 3 ports not active nor detect nor detect a working USB memory stick when inserted double check that 5F85N enclosure is not bent or warped causing a mechanical poor connection when outside Dell case.  to test this, try move all components back to Dell case but do not insert 5F85N back into Dell front bezel.  Leave it hanging loose like you did on a test bench.  If problem reoccurs, it would provide a clue that when you install 5F85N in the Dell front bezel, it somehow corrects itself mechanically. A user once reported: "The face plate had been removed and not replaced correctly with the center bottom catch not seated, causing the plate to bulge out and prevent a clean connection when anything was plugged." Solved: Optiplex 9020 USB front ports (only) not recognising devices - Dell Community    
Hi Folks, As part of a case swap for my old Optiplex 9020 MT I have removed the motherboard and a selection of other components (PSU, CPU + Cooler, case fan, thermal sensor, Power -On button, RAM, etc) including the IO-USB assembly (part no 5F85N ).  When I re-build the PC on the bench outside the case the PC successfully boots into Windows 10 without any boot errors (so no F1 to continue errors).   However, when I test the IO-USB assembly I find none of the 4 USB ports are live/work correctly.  Testing confirms that:- 1.  no "IO cable failure" error message on boot    2.  HDD LED on the assembly asserts correctly (flashes when the disk is accessed) 3.  Audio-Out jack on the assembly works correctly (headphones play sounds correctly) 4.  both USB 2 ports not active nor detect a working USB memory stick when inserted (even though HHD LED/audio jack OK from same MoB header work) 5.  both USB 3 ports not active nor detect nor detect a working USB memory stick when inserted (from a different MoB header) 6.  when I re-fi the IO-USB assembly back into the 9020 MT case all the USB ports work correctly!! Does anyone know of a reason why the USB ports do not work when the IO-USB assembly is not installed in the PC case (but is correctly connected to a working motherboard)? Many thanks Jack        
Dell Optiplex 9020 MT : IO-USB Assembly : USB Port Issue when Outside PC Case