My Hard Drive is a Retail Plus 500 GB(465 with formmatting blahblahblah) and I run it on windows 7 home premium.Usually it runs great.But today when i plugged it in nothing happened.Literally NOTHING.The harddrive didnt vibrate,and the laptop didnt recognize it either
If it isn't spinning up, it is either not receiving enough power, the PCB attached to the hard drive is damaged or it has mechanically failed. Try on a different USB port, and on a different machine. Try using it with a different USB cable, and try on a powered USB hub. Sometimes a laptop doesn't provide quite enough current to get the drive spinning, so try on a desktop PC if possible.
As hard drives wear the mechanical motors that spin the platters become worn and inefficient - do you hear any noises coming from the enclosure? I have one external that is old and worn out, and will only spin-up if positioned vertically, or if I gently rock it back and forth.