Drivers are used by Serato DJ to communicate with the Rane hardware. To run Serato DJ on a Windows PC you will need to make sure you have the correct drivers installed in order for the hardware to be recognized. If you do not have these installed you may receive the "hardware disconnected" message.
If you have your Rane hardware connected when installing the Serato DJ software the drivers should be installed automatically. If not you will need to connect your Rane hardware (Serato DJ will need to be closed) and run the driver updater tool.
This can be located in this location on Windows 7/8:
Start Menu > All Programs > Serato > Serato DJ > Update Drivers
Or if you are running Windows 10 it will be in:
Start Menu > All Apps > Serato > Update Drivers
This will open up the Rane driver updater tool, detect your connected hardware and install the relevant drivers.
If you are having trouble installing drivers it is probably due to your machine not being able to recognize which piece of hardware you have connected. Try changing out the USB cable, switching USB ports on your computer and make sure that you are not connecting through a USB hub or splitter.