Leakage current from switchmode power supply
 

I have a PC and an external HDD switchmode power supply both connected to
the UPS battery backup outlets. The external HDD supply is ungrounded but
the PC is grounded. There is leakage current between the USB connector of
the external HDD and the PC case. It lights a neon tester when touched
between these two points. The rated leakage current is 250 microamps at
maximum voltage and frequency. Normal AC power uses true sine-wave but
battery backup uses modified sine-wave similar to square-wave. Does the
modified sine-wave cause higher peak leakage currents when switched to
battery mode? I don't have an oscilloscope to see what the leakage current
looks like while on battery. Can this higher peak current cause a shock
when touching the USB connector and the PC case at the same time while the
UPS is on battery mode? When the USB cable is held next to a t-coil
equipped hearing aid while on battery there is a buzzing sound which does
not occur while on AC mode.