I'm talking 3m. How long does a usb cable have to be to be too long to supply power?
It depends on the demands of the thing (in this case the WD Passport) requires, but the base amount that can be supplied by a USB is
limited and degrades over the length of the cable due to the innate resistance of the cable, so the power is lost over the length of the cable.
The WD should have come with an appropriate cable though.