First of all, here's a similar issue we had a week or to ago (see my reply - should be of some use hopefully!)
Secondly, Because its a pre-built, they probably have had it rigourously tested, and probably have put the bare minimum PSU wattage in there to get the job done, guaranteed (to save money!)
Personally, If i was spending money and putting a nice HD in there, I would get a good brand PSU, to make sure nothing gives way, with the additional wattage. (My general rule is, I wont have a $90 piece of Hardware on its own (not including CPU, MOBO Etc) being supplied by a crappy $20 PSU)
If you wanted to go ahead and carry on, you would probably just be ok though - as long as you arent experiencing any PSU issues currently.
Do you really need a TB drive though? Worth re-thinking perhaps? If you do, thats fine, and go with suggestions above.
Disclaimer: If anything blows up - don't blame me.