Laptop switches off when HD attached

Hello,

my laptop is an Asus x58L series. a few weeks ago it randomly shut itself off whilst i was working and would not switch on again. at first i thought it might have been dust in the battery and taking it out to clean it worked but only once.

i followed some advice on this forum about taking out the power cord and the battery and the laptop switched on again - hurray! the issue now is that it will not switch on if my external harddrive is attached to it or if the laptop is already on, it automatically switches itself off.

can anybody help me with why this might be happening or what i can do to resolve it?