Sounds like you have an old WDTV Live / Live Plus and your TV is only compatible for Composite Input for this device.
The cable you need is a 3.5mm to Composite (Yellow,Red,White)
Or if your TV SCART is Component Input then you will need 2 Cables (Green,Blue,Red for Video Signal + Red,White for Audio)
Component will give you much better Picture Quality over Composite
Unfortunately these are a *special* cables with different pinouts to a standard cable AV cable.
This post … the user lost his/her cable
Bill_S recommended contacting WD Techincal Support for a Replacement
http://community.wd.com/t5/WD-TV-Live-Live-Plus/Replacement-composite-cable-help/td-p/33196
