USB extension drive problems

Hi, a while ago i got a new Seagate 500GB USB extention drive for my desktop comuter and it has worked since but now when i plug more than 2 other more drives into my computer the extension drive stops working :doh:. I then unplug it, resart the computer and everything is there :thumb: but then i can’t use and other USB drives :doh:, back to square one. Is there any other way of putting more than 2 USB drives into to the computer? any help would be great!