Sorry but I don't agree with you at all
The most interesting thing is that you start saying that "you have to seriously take this scenario".. but your arguments aren't serious at all... MS CAN and, if it wishes, WILL BAN anyone that used a modded HDD... That's simple!
If you really think seriously, this is like saying that Dell would still give you warrant for computer HDD that have been replaced for and HDD from other manufacturer! Do you really think MS would avoid banning just because of that?
You wrote that someone would purchase a hacked HDD without knowing and that's the reason MS wouldn't ban the Console. May I Ask you WHY? I think this would actually give them more reasons to ban the console for several reasons:
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1) Maybe you don't know that you have a hacked HDD and THAT's a problem. There may be several "manufacurers" that are using this method to create fake 'original' 120 HDD. If MS doesn't start acting now, maybe it will have several problems latter. Banning is a way to stop these "manufacturers" from doing so...
So banning HDD will prevent this "manufacturers" from filling the market with non certified HDD.
"Hey.. I was banned because I didn't know that this was a hacked hdd. Let me come back to live".
"Sir, we can do that, but do you realized that you purchase a HDD that MS will give no Warrant?"
"Well.. I..."
"Also Sir, if you tought this was an original MS HDD, do you realize that you payed US$ 120 in a lower quality HDD that you could purchase anywhere for US$ 40,00. A Western Digital HDD is 10 times more likely to have problems that the original MS one. So we can solve your problem, but the seller sold a cheaper HDD to you for three times it's real price..."
".. Three times the real price and WITH NO warrant service from MS".
"Now may I ask you again: Do you want to get mad to MS about being banned from live OR do you want to go back to the seller and have your money back"?
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2) Also how about the warrant? If this is a hacked HDD and the owner doesn't know, he/she will probably sent the HDD to MS for repair in case of problems. Following your argument, MS would than replace a fake HDD only to make people happy. Imagine that:
"Here.. this guy sent us a hacked HDD that is MS original! But maybe he didn't know... so let's give him a brand new one with 3 years warrant!".
"OKAY!!!"
MS SHOULD make you know that you have a hacked HDD so that you must decided if you want to use a hacked HDD with the risk of ban and without the warrant service.
Imagine if it was a car:
"GM, may I help you"?
"Hey I just purchase a corvette and you guys don't want to replace some parts using the warrant service"
"Sir, we checked your plate, and your car is a stolen car. Also it's 7 years old, and lastly it's not a corvette. It's actually a mustang"
"But I didn't know that!"
"Oh, sorry. We will offer full repair service than! Please don't be mad at us"
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3) Selling this HDD like an "original" MS is a scam, and there is no reason why MS should support it anyway. It's like some fake mp4 that some sellers are selling on ebay. Do you think apple would give support and updates to a fake iPod? Why?
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4) So you got a Hacked HDD and you don't know! So banning will be the way to know it. If someone call MS complaining about not acessing the Live, they will simple tell them they have a Hacked HDD. Simple as that
" I want to know why I was banned"
" Sir, we realized that you have a hacked hdd that's not original from MS"
"But I didn't know that"
"Sorry Sir. We recomend you contact you seller and ask for a full refund"
"For just a HDD?
"Sir, you purchase a product from a seller that gave you a hacked HDD as a an original. Are you really sure that this HDD was the only thing wrong? Maybe he sold you a refunished RRoD console saying that it was "brand new". We REALLY recomend you going to the seller and asking for a full refund for the HDD AND console".
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5) MS is not a govermental company and have no obligation to make their serive avaiable to anyone. They CAN ban you anytime they want. Have you read the agreement to enter in the Live? Ms is a private company and they can decide which one will use or not their service..
How many stores have a sign saying: "we reserve the right to not serve xxxx" or "we do not serv lunch after 12:00 PM"? Will you sue McDonnals for not serving lunch at 06:00 PM?
Also MS is smart enough to connect your live account to a user, not to your console. So they can ban your console without having to refund you the payment.
Is like police towing your car because you have a broken headlight. You can use the car because it have a problem but you stil have your license. You can drive other cars....
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6) Also is naive to think that this is 'just a hacked HDD'. Do you remeber the 'broken window theory back in N.Y?' The one that said that someone who broke a window today will be commiting murder months latter? So the best way to prevent a murder is to give a hard punishment to a broken window and prevent the thief to become a murder.
The one that uses a hacked HDD today will probably try a hacked firmware next month. If they punish a 'small thing' like a hacked HDD they would prevent people of trying other more 'advanced' hacks.
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7) THERE IS NO GAIN for MS to allow haced HDD. A person with a Hacked HDD is more likly to try a hacked firmware, so the changes are that this person will use backuped games which won't give no return to MS. There is NO support to the theory that hacked HDD will give more "return" to MS for downloaded games or content. This same person that downloaded "US$ 5,00" in content have 40 backup games that costs to MS arounf US$ 30,00 in bandwith, servers and maintainace for the game servers on Live.
Does anyone have never though how much backup games cost to MS? Not only the 'direct' loss of the game price, but the amount that MS spend on maintainace of the servers for thousand of players that don't purchase the original game?
Banning hacked consoles would give them less consoles acessing the Live, what would give them less maintainence what would turn into saving.
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Last.. why do people always thing that MS will ban everyone in the day after the Hack have been discovered? Does anyone realized that any change or fix on live requires money and time? They will probably have to create a new search program that will have to search every console over the net to send additional data about the HDD. This will cost salary od developers. Also, this "small" change would give them additional 2% of data to be sent and downloaded back to their servers. Depending on how many consoles are there connected to live this could means several TBs of storage and several additional servers for the additional data transfered.
And remember: This would be additional cost and changes that they would have to make just few months before the launch of a new dashboard? Do you think that the directors would aproove this additional cost so close to a big change? Wouldn't be cheaper to just add this changes to the new dashboard?