Two 1TB My Passport on Win 7: Second drive not recognized

I have two new 1 TB “My passport” external USB drives (USB 3.0). Both work properly working while plugged in alone. No matter in which order I plug them to the notebook (I have only one USB 3.0 plug): The first hard disc would work as expected, the second will not be displayed under “my computer”. Instead there appears a useless “drive f:”, which would not open when clicked upon (error message: “wrong parameter”). Power supply seems ok, LED on drive burning.

When I click on “remove device” both drives will be displayed, the first one with a drive volume letter, the second without the letter and both can be removed properly.

So the system seems to recognize both drives to some extent, and both are working (if, and only if  plugged in as the first drive, no matter whether to an USB 2.0 or USB 3.0), but not together.

What can I do? OS Windows 7, notebook acer, < one year old.

It’s a Windows problem see post 11