What made me think of the question when I posted is that with 2 piece cd players I've always heard HUGE differences between digital cables.There are many reasons why large differences can be expected between digital cables that connect CD transports to DACs. Not only the type of cable, but even the length of the cable can make a major difference. See this paper for starters.
Basically, in those applications the effects of impedance mismatches between the cable and the components it is connecting, the introduction of noise due to ground loop effects or RFI pickup (both of which are cable sensitive), and other ways in which the cable may affect waveform integrity can ultimately have very significant effects on jitter at the point where D/A conversion is performed within the DAC (to a greater or lesser degree depending on the jitter rejection capability of the particular DAC).
None of those factors have any relevance to transmission of digital data from a USB hard drive to your Oppo, as long as nothing involved is defective. The technical considerations are completely different, and vastly less critical with respect to their potential effect on sonics.
Regards,
-- Al