I have an external 500 GB HD. I try to plug it in

There are two types of hard drive.

1. which can be connected dircetly to the USB port and do not need any external power supply to boot. These type of hard drives are similar to those we use in Lap Tops.

2. The second type the one which is similar to those we have in desktops. For these we need external power supply for it to work.

Please let me know which type of HD you are using.

For that make sure usb cable works fine.
Try with different usb cable.
Also connect usb cable in different usb ports of computer.
Right click on my computer then click on proeperty then hardware->device manager.
Then click on sign of usb controllers then uniunstall all usb drivers 1 by 1.
then restart computer.
Then check.
If still not recognized then when you power up the computer press F8 key then enter in safe mode.
then check.
in normal mode again.sometimes harddisk need to start in safe mode .
Let me know if you need more assistance.
thanks.