Another way to explain this....
With a CD transport and a DAC, the birth of the PCM audio stream starts when the data is lifted off the disc. With the BDP the birth of the PCM audio stream is right after the point when the software writes this data to the sound card buffer. From that point on the data streams share common issues like jitter, noise, etc, which cables have been known to influence.
The accuracy of the cd transport to read this data on CD is on the same playing field of how well the BDP's open-source MPD software (and libraries) can convert the file to a PCM stream and constantly write that data to the buffer without any underflow. As an example issues with a CD can be scratches, disc rot, unbalanced or misaligned hole, and all easily detectable and it's really not unlike vinyl. For the BDP, like any computer based player, issues can be CPU load, IO load, drive faults, memory faults, etc., so a USB cable between BDP and HDD won't amplify the signal when it's transferred as it's still just a file. It's no different than the mechanism used to copy the file there to begin with.
Like i said before, that doesn't mean you won't benefit having a longer vs shorter usb cable, hdd vs ssd, or using a bus-powered mobile drive vs a wall-wart desktop drive. Component isolation, audible HDD noise, or a switching power supply will play a much greater role than which brand of USB cable I use with my HDD.
