I have two hard drives, one with a USB type A and one with a USB type C connector. I also have a USB type A to C cable.

Connect the hard drive with USB type C port with the USB type A port of my computer works well.

But connecting my hard drive with the USB type A connector with the Thunderbolt USB type C port of my computer using the same cable doesn't work. The hard drive doesn't power up and is not found by my computer.

Replacing the computer, cable and hard drive didn't help.

Why doesn't it work?

As it turned out, when I use a regular USB cable with an USB type A to C adapter, the USB type A drive connection works. But the two other cables that came with the hard drive and with a charger don't.

  • "Thunderbolt USB type C port" - Thunderbolt and USB type C are two different standards, you'll have to clarify what you mean (use the edit button). – gronostaj Jun 13 '18 at 13:36
  • Thunderbolt refers to the bus. Type C to the connector. That should be obvious because otherwise it doesn't make sense. – Frank Breitling Jun 13 '18 at 13:42
  • Make and exact model of PC? – Moab Jun 13 '18 at 15:18
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    @gronostaj see this...cnet.com/how-to/… – Moab Jun 13 '18 at 15:40
  • @Moab It is a HP Spectre x360 Convertible 13-ae002ng with Thunderbolt 3. – Frank Breitling Jun 13 '18 at 20:53

Let me sort out your setups.

  1. You have HD1 with Type-C port, and, when connecting it to Host PC with Type-A port using Type-A to Type-C cable (called "legacy cable assembly"), it works. Explanation of why: the Type-C end of legacy cable assembly has embedded resistors on CC line, normally 56k pull-up to +5V VBUS. This connection defines the Type-C end as USB host, so USB Type-C device accepts it as host, and works fine.

  2. You have HD2 with Type-A receptacle, which is technically illegal USB arrangement. But there were some industry hacks who used this connection, and usually supplemented their HD product with proprietary cable with Type-A plugs on both ends. Now, if you try the same cable as in #1 case, the cable C-end still has the "host signature". When you plug this cable into Type-C compatible port on another host computer, the cable says "it is host", and the host says "it is host". So plugging two hosts together results in inactive connection, and PC host won't assert VBUS. So the HS2 will have no VBUS and won't start nor connect.

  3. When you have a "Type-A receptacle to Type-C plug adapter" (I assume this is what you have, something like this one),

enter image description here

this adapter has CC pull-down (5.1k), which signifies it as "USB device". As such, the adapter instructs the Type-C port to be HOST, and the port will output VBUS. Thus the adapter turns your Type-C port into a regular Type-A old USB port, with VBUS turned ON. If then you use your proprietary A-A cable, the HD2 also works.

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    I’m always glad to see an Ali Chen answer on a USB Type-C question. Thanks for sharing your deep expertise. – Spiff Jun 14 '18 at 4:49
  • Thank you very much for your in-depth answer. It surprises me, that a Type-A to C cable is different from a Type-A cable with a Type-C adapter. Is this a necessity or just a possibility or could it even be vice versa? – Frank Breitling Jun 14 '18 at 6:24
  • @FrankBreitling, in case of normal C-C cable, the role of device propagates to host port over CC wire, making the mated role selection. There is no pull resistors in the cable, it is symmetrical. For "legacy assemblies such as Type-C to Type-A RECEPTACLE (aka OTG adapter), and for Type-C-Type-A PLUG, the "role signatures" have to be embedded into Type-C overmolds, since Type-A connectors don't have these role-defining pins, and can't do it automatically. See also electronics.stackexchange.com/q/255684/117785 – Ale..chenski Jun 14 '18 at 14:07
  • @AliChen Thanks, for your further explanations, but I am still confused. If I understand you correctly, the problem with some cables is a 56K resistor which makes the type C plug a host. But why is this resistor there, if otherwise the cable would work as expected? – Frank Breitling Jun 14 '18 at 21:20
  • @FrankBreitling, why the resistor is there? Because a device with Type-C port is expecting a pull-up, and the value of pull-up determines how much current a device can take. If it doesn't want any current (as, say, self-powered HD), this is still required by Type-C protocol, and a normally-designed USB-compliant port will refuse to work without proper protocol. – Ale..chenski Jun 15 '18 at 1:52

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