Usually it's the problem, as you get a loop caused between:
Mains ground on Printer Supply to Mains ground on PC.
Mains Ground on PC to DC ground on PC's USB.
USB cable from PC to USB cable on RUMBA (or any USB based electronics)
DC ground on RUMBA to Mains ground on Printer Supply.
This forms a loop. If instead of these all being the same voltage level, there is instead a voltage difference at one point (eg: Say 1V between Mains ground on PC to DC ground on PC's USB), then you get current flowing around the rest of the loop "trying" to make them equal. Depending on all the bits in the way, the result could be erratic behaviour, failure of things to work, or cables getting very hot.
With the PSU's off, you could measure resistances between AC ground and DC ground. When on, you could try measuring DC (or even AC) voltage. A lot of people recommend shorting DC ground and AC ground on those dedicated Brick-style PSU's, but I'm hesitant unless it's proven that doing so is actually safe (especially when many of these dedicated PSU's are of unknown origin or compliance, as they're usually made and sold on the cheap).
Mains ground on Printer Supply to Mains ground on PC.
Mains Ground on PC to DC ground on PC's USB.
USB cable from PC to USB cable on RUMBA (or any USB based electronics)
DC ground on RUMBA to Mains ground on Printer Supply.
This forms a loop. If instead of these all being the same voltage level, there is instead a voltage difference at one point (eg: Say 1V between Mains ground on PC to DC ground on PC's USB), then you get current flowing around the rest of the loop "trying" to make them equal. Depending on all the bits in the way, the result could be erratic behaviour, failure of things to work, or cables getting very hot.
With the PSU's off, you could measure resistances between AC ground and DC ground. When on, you could try measuring DC (or even AC) voltage. A lot of people recommend shorting DC ground and AC ground on those dedicated Brick-style PSU's, but I'm hesitant unless it's proven that doing so is actually safe (especially when many of these dedicated PSU's are of unknown origin or compliance, as they're usually made and sold on the cheap).