My laptop cannot Autorun my external HD but it recognizes the device

My external HD (Seagate) is recognized by my laptop (XP Pro) but the drives won’t show. Worst, I can’t run applications (like excel)when the HD is connected to my laptop. It doesn’t Autorun either. I tried to use my HD casing to other HD, it worked. I also used another casing to my HD but still doesn’t work. Please help!