Computer will not boot up once HDD is attached

Hi everyone,

I have an external USB Seagate HDD that seemingly died during transit to my new home. It worked fine before leaving and was completely dead when it arrived. It was packaged well (very fragile Venetian glass made the same trip intact) and there are no visible signs of damage. However, it was completely dead. NO lights, sounds or movement. I've taken it apart at the request of Seagate and removed the HDD and connected it straight to my motherboard as it is just a standard SATA HDD. However, when I turn on the computer I get a green power light for a split second and then everything switches off. I need to remove the HDD and then unplug the computer before it will boot up again. I have tried this with 2 computers with the same effect so it can't be the PSU. The HDD does not not even need to be connected to the MB for this to happen, just the power supply, so it's not the MB(I have tried connecting it just to be sure but it's the same result). Both computers work fine with their standard HDD reattached. Does this mean my external HDD is completely dead? Would a data recovery store help? It's got some very important personal files on it so I'm desperate to get it working again. I've searched other forums but not come up with any answers, so any help would be greatly appreciated.

Regards,

Dave