LOL, I'm from Portugal too
Bastard, I bought mine from Staples for 1000€ 4 months ago... I must say you made a great purchase. The thing is: mine doesn't show the lag issue (did 4 times on the first 3 weeks, then I updated the VIA driver, and since then I never got anymore brrs (again, this is not a solution). Could it be that the store is aware of this problem with the laptops and knew your system had the lag issue? I'm saying this because they have another M3438 for sale on their site, for 1529€, a little more pumped up.
Anyway, I understand your situation. I'm going to be honest with you: when I purchased mine, if I noticed it showed frequent signs of lagging, I think I would have returned it. Even if I purchased it cheaply, I think I have the right to get a fully working product. Even if I only paid 100€ for it. It's up to you to decide wether your lag is tolerable or not.
Regarding the external hdd questions:again, I can't assure it's a 100% solution, I would need to get more feedback from users that have tried it. In theory, yes, it's the solution.
-it's natural that Windows performance will not be the same than having it installed in an internal hdd. Again, I'm not sure how it would perform, even though II have an external hdd, I only have there my games, music and movies. Windows is installed on my internal hdd. But if you get a good hdd, with a nice buffer and rotation speed, I don't think you would notice much loss (when you get used to it you won't even notice).
-I think the USB 2.0 transfer rate is more than enough for about everything I can think of. I've read many comparisons between USB 2.0 and Firewire, (480 Mb/s vs 400 Mb/s), and it seems the only Firewire advantage is that it keeps a more stable and constant data transfer, unlike USB that seems to have some drops. But I don't think this should be a problem, even with drops, the available transfer rate is way superior than today's hdd maximum data transfer rate, so there's no problem here.
-Yes, it's perfectly possible to live with only an external hdd. Afterall, it's a hdd like the others, you're just giving a break to the internal one.
My final thoughts are: explain your situation to the store, and ask them to replace it with another one. Maybe, if you're lucky, you'll get one that doesn't show the symptom (even though the problem is on all systems, on some it doesn't show itself, it's like a virus :S) .Then, think very well wether your lag is tolerable or not. If not, you're paying for a product that isn't in good conditions, and that's not right, even if cheap. If you think it is tolerable, then go for the external hdd and get yourself a great system for a fantastic price.
By the way, this really is a great laptop if we forget about these lag nightmare right?