Correct answer: there is no drive letter or path! You need to assign a number, like H,I,J or something like that. Pretty much the same concept as a disk drive letter!
Methods: Win7, computer right-click - Management - Storage, Disk Management - find the USB flash drive- -In the following box "click" to select, right-click - change the drive and path - select a disk drive letter, OK, ok! Look at the USB flash drive in your computer.
Note: This problem is not due to the driver! It's not the same as any other driver problem. It's the fact that the driver has been installed correctly, but the portable device has a "yellow exclamation mark"!
I hope you will support this!