Computer does not recognize external hard drive

I am running XP on a Dell E310 with 2 USB ports on the front and an extra 4 that I added on a card in the back.  I have 4 external hard drives for backup.  When I have 2 connected in the front and the other 2 in the back, the computer will not recognize the 4th drive.  One of the two in the back is connected to a USB port that came with the computer and the other is connect to one of the 4 USB ports I added on a PCI card.  The light comes on for the one that is not recognized and the computer will recognize it if I connect it to one of the two ports in front.  I even tried another of the 4 ports in back with no success.

If the Passport is not getting enough energy it wont be recognized by the computer. But try this, go to the disk management windows and check if all the passports are recognized there, if they are, one of them should be offline, right click on it and select the option to make it online. Check the link below for the steps to get to the disk management windows.