|External hard drives often require the USB port you plug them into can provide the max 500ma the standard USB specs provide for.|
If you use a hub to connect a single external hard drive to the laptop, or especially if you connect two or more external hard drives to a hub that connects to a single port on the laptop, you are likely to have problems because the connection can't get enough current, and you may get error messages when you do.
If the external hard drive is 2.5", some come with (a) sufficient cable(s) to allow you to make two connections if one particular USB port can't provide enough current, some don't.
Also, some laptops can't actually provide the standard max 500ma from any USB port, and/or if you are plugging into a port in USB 2.0 controller PC Card (PCMCIA card) , it may not be able to supply the full 500ma to each port unless you plug in only one external hard drive and nothing else into it.
For more info, see response 3 in this:
and see response 1 in this starting at
"An external hard drive requires more current from the connection to the computer than most devices do."