Ylod with harddrive in system?why sony, why?

ive got myself a fat ps3 ( dont know the model number, has no sticker, but i can tell you its one of the later models, 2 usb's) and everytime I try to run it, i get the ylod. done a reflow on the cpu and the gpu, tried harddive swaps, power supply works. Weird thing is, run perfectly fine for hours without the harddrive, but as soon as you try to run it with the harddrive, ylod. ive got a 120 gb and 160gb, both work in my other ps3, but not this one.