A cheap PSU with GND at a higher voltage than mains GND caused the other similar case. Please check for a differential voltage between (-) on your PSU and your PC case.
Traumflug: There is no way to connect USB 5V to the PSU 5V on RAMBo. It is jumper select. VCC is sourced from either USB or the onboard SMPS.
Traumflug: There is no way to connect USB 5V to the PSU 5V on RAMBo. It is jumper select. VCC is sourced from either USB or the onboard SMPS.