dvd writer is playing up and only seems to like writing dvds (not CDs)

in my PC I have 3.5" disc, DVD ROM, DVD writer and ive just tried to plug in a CD writer - piggy backed off the grey wide connector and a power input...

but when i turn the PC on... the DVD writer and CD writer are not available and wont install etc (these are on the shared lead)


why is this and is it possible to do what im asking?