Is there Javascript based mechanism with which i can detect, through a browser, that a user has a specific usb device plugged in?
For security reasons, JavaScript in web browsers provides only restricted access to computer resources. It is not possible to store files in an arbitrary folder, start applications or communicate with a USB device.
But there are some exceptions :
Your javascript code is sand boxed and therefore cannot access system resources such as usb devices.
I think the only way to access the USB devices would be to write a browser plugin. How to get USB devices List from Browser