I have two PIs: Pi 2 Model B v1.1, and Pi 3 Model B.
Both Pis are running with defaults. Also, both Pis are each powered by a 2A supply.
I also have an externally powered hard drive dock that happens to be USB 3.0. I understand that neither of my Pis have USB 3.0 support, that's not the issue.
I discovered that if I plug the dock into the Pi 3 using a USB 3 cable, the drive is not recognized by the Pi (no new device shows up in dmesg or lsusb).
However, if I plug the dock into my Pi 2, even with the USB 3 cable, it is recognized and I can even mount/access the drive.
Since I needed the disk to be mounted on the Pi 3, I was able to mount the drive by instead using a USB 2.0 cable.
The question: Why would the USB 3 cable work with the Pi 2, but not the Pi 3?