I lost the power cable to my WD external hd when moving. So i bought a new adjustable cable and plugged that in. The light comes on just like it should but it will not register that there's any device plugged to my computer. Ive tested my USB ports and tried the external on my ps3 and its definitely that which has the problem. I may have put in the wrong polarity when wiring it up and this may have fried the drive, this is all I can think of, however I would imagine that the light would break to and the wiring would stop if this was the case
More about :changing power supply external hard drive light
Is this a 2 1/2 in drive or a 3 1/2 in drive.
When you say the new AC adaptor can be adjusted, You must also look at the current rating. For example (3 1/2 " Drive) requires +12 V @ say 2 Amps, if your replacement ac adaptor is for 12 V at 0.5 AMps then you are loading it down and the output voltage will be low. Most harddrives are from around 5 ->12 Wats. Watts /voltage = current requirement. A 10 Watt 3.5" HDD requires just under 1 Amp. the same 10 watt drive in 2.5" requires 2 Amps (uses 5 Volts)
You are correcct, if you revearse the Polarity the LED should not lite.
An LED will light with reveared polariy provided the voltage is high enough so that the LED can say OUCH, and BUY - It will ligth one time as it burns out.