Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Have you tried putting the end that wouldn't go into the laptop into the HDD? Might as well check it's the laptop and not the cable!
 
I am not sure how but it fits now. I plugged the end in couple times, into the drive it might of loosened it.
 
This happened to me at work before, one of our firewire 800 cords had plugs that had a narrower indentation on the top of the plug than the firewire 800 port on the mac pro we were using. It was kind of the odd man out because we had alot of other firewire 800 cords around and they all fit, but comparing them did show that the top indentation was too narrow to fit in the mac pro's port on the problem cable.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.