|Your external harddrive may not work properly when plugged into certain USB ports.|
E.g. people often attach USB hubs to laptops, and there are many USB devices that won't work properly when connected to one.
See response 3 in this:
In addition, all 2.5" external hard drives require the maximum 500ma USB spec be available from one or two USB port(s) it is plugged into. Some 2.5" external drives have two connections you need to make to the USB ports on the computer in order for the external drive to get enough current, if the drive can't get enough current from one port.
Does your laptop have USB 2.0 capability?
If it doesn't the external harddrive will run much slower connected to a USB 1.x port.
If it does have USB 2.0 capability, do you have the USB 2.0 controller drivers loaded?
If you do, in Device Manager - USB controllers you should see at least one entry "Enhanced USB controller" or similar .
If you don't see that, you need to load the main chipset drivers for the mboard in order for Windows to install the USB 2.0 support