I have an HP Probook 4420S which is only a few months old. I have a Seagate 1TB Ext HD via USB. It worked fine for a few months and then the Laptop would shut down. I noticed that the fan would speed up and then the whole Laptop would just go blank. After testing I have found that the Ext HD is causing the problem. It is not at a point where I can just plug in the USB cord from the HD and the fan kicks into high speed and within about 60 seconds the computer shuts down. Any ideas what is causing this? I really need to access the information on the HD.
I am assuming you have plugged the extremal HDD into a wall outlet and turned it on before plugging the USB cable into the laptop. If so, it could be the power adapter for the external drive has failed or is not working at 100 percent. This could result in the external drive trying to draw more power from the USB port than it can supply. You could try a different USB port on the laptop, but they are all likely on the same power circuit.
You could get a spare power adapter for a few dollars at most electronics type stores or on line.
You might also Google Seagate followed by the model number and the word "problems" to see if others have had the same issue.