my laptop not detecting (only light is on) external hard drive but pc does

If the drive is detected fine on a desktop, but not on a notebook, it can be a power issue. The USB port on the laptop may not be delivering enough power to the drive. Make sure the laptop is plugged in, and not only on battery power when trying to access the drive.

There are also cables that have plug into 2 USB ports on the laptop, as well as the drive, to deliver extra power in these types of situations.
 

Paperdoc

Polypheme
Ambassador
digitaldoc is right. Here's a clue to help figure it out. USB2 ports on any machine have limited power to send to any device. This is true no matter whether the USB2 port in on a laptop or a desktop. However, some laptops do not quite send out the power they should, but some desktops can send out more that is required.

Another clue: external drives specifically called Portable Drives and designed for use with laptops are designed to work with the low power available from a USB2 port. But almost all larger external drives designed to work with desktop machines need more power that they can get from a standard USB3 port. They can only work if the are connected to a USB3 port, OR if they are connected to TWO USB2 ports with their special cables, OR if they have their own extra power supply cord plugged in.