On Tue, Oct 06, 2015 at 09:45:33PM -0400, D. Hugh Redelmeier wrote:
My daughter has been using a 2T 2.5" external Seagate Disk. It has a USB3 interface, compatible with USB2. She uses it with her ThinkPad T520 which has only USB2 ports.
A couple of weeks ago, the disk stopped working. The disk would power up but it would not show up. Oh oh, another disk failure.
But it worked when I plugged it into my computer. In fact, it worked with any USB3 port and no USB2 port (even with a powered hub).
I thought that this was impossible. So did Seagate support (I was talking with them on another matter -- two dead drives, one of which they had claimed was out of warranty when it was not). The support guy suggested trying a different cable, but neither of us thought that it was likely to work.
Changing cables did work.
So: a cable can fail in such a way that a USB3 device won't work with a USB2 port but will work with a USB3 port.
The connector on the drive itself has a funny B shape. It is called a USB 3.0 micro-B socket. Apparently a USB 2.0 micro B plug can fit in half of it! Curiouser and Curiouser.
I wonder if an ordinary micro USB 2.0 cable, as used with cell phones, would connect the drive to a computer's USB 2 port.
Well in USB3 mode, you are transfering data over the two USB3 pairs, while in USB2 mode you are using the original bidirectional USB pair. So USB3 ports would do most of the communication over different wires than a USB2 port on the same cable. I believe the original USB pins are used for negotiation and detection, but that doesn't involve much data or very fast, so if the wires for the old USB are damaged, it might still work to detect the disk before switching to USB3 mode on the non broken wires. -- Len Sorensen