The question above is burning in my head, and when I google it, I couldn't any satisfying result to my question. I even couldn't find the right image of the USB Y-cable in the search result. See the cable picture of the Y-cable below.
In this article I hope can answer the question and let you know its purpose and how to use it. In my case, I use it to connect a Prolink PUH302 USB3.0 4-Port Hub. It comes with the USB 3.0 + USB2.0 Y-cable. In its limited documentation which only available in the outer box, shows single wire USB cable which is different with what I got.
When you connect a device (like the Prolink PUH302 USB3.0 4-Port Hub) to a USB port on a computer system, this device is drawing power from the USB port to power the device. If the device draws too much power from the port and the port cannot provide enough power to the device, the device may not work correctly or may exhibit symptoms like random disconnection, device/drive clicking noise, device not detected, file transfer problem, device will not spin up and more.
One way to provide more power to the device is to use another USB port to help supplement the needed power for the device.
What we have done is use a regular USB cable and adds another USB connector to the end that plugs into your system. It looks like a Y cable. The two tip of the Y-cable are the two type A USB connector connecting to two USB port on your system and the other end to the device (USB hub device). You will notice on the Y cable, one of the type A connector will have a cable that is thinner, that is the supplemental USB power connector.
HOW TO USE THE Y CABLE
First connect the connector to the enclosure then the thick cable with the USB header to your system. If you notice problem symptom listed above, please connect the supplemental USB connector to another USB port on your computer.
1) If I am using USB 3.0 device and port on my system, why do I still need a USB Y cable, since I know my single USB 3.0 port do provide enough power for my external drive?
The USB 3.0 Y cable actually serve two purposes, if you are using it with a USB 3.0 port on the system, you will only need a single port or connector. You do not need to use the supplemental USB connector. If you do not have access to a USB 3.0 port, this USB 3.0 Y cable will function just like a USB 2.0 Y cable and allows you to connect your external drive to the USB 2.0 port on a non USB 3.0 system and still have access to you data.
2) Will using both connectors on the Y cable overload and hurt my drive?
No, it will not. It is safe to use with both connectors.
3) Can I use only the thinner cable?
No, the thinner supplemental connector is only for power and will not provide any data signal.
4) I am connecting my external enclosure using eSATA connection and I also need to use both USB connectors of the Y cable?
You MAY need to use it IF the external enclosure draws too much power from one port. By using the main with the supplemental connector, it will have enough power to power the device correctly
What is the purpose of a USB cable with a Y connector