You have to log in to the original PSN account first to allow you to use it. You can't use it on separate accounts or sub-accounts.
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The "I have a life" excuse may be tired, but so is the "THIS IS THE WAY THE GAME IS PLAYED, EVERYTHING ELSE IS CHEATING!!!1!!!" screed that pops up everywhere.
I say this because I do have a life. I used to grind 24 hour races all by my lonesome, leaving my PS2 on for days as I had to sleep, eat and bather (and sometimes only two of three). I stuck through two iterations of GT doing this.
Can't do that now. I have a family and a job. I did a 300 kilometer enduro the other night, and at the end of it, (four hours... had to help cook dinner and eat in the middle of it) I had to apologize to my wife.
Between working 8 hours a day, moderating 4-5 hours, helping my kid do her homework, doing the groceries, fixing things around the house, doing car-testing (IRL) on odd weeks and weekends and spending time with my wife... actually playing for more than an hour or two a day is a serious strain on our marriage. Which is why my first enduro (done just so I could reassure myself I could still do them) is going to be my last for the next few months.
Personally: Meh. They'll get it... won't be able to sell cars off the save at the same time because of the trade limit. Use it to play with a few cars, and get sick of it and move on. I can't ever see myself copying a game save. It's not terribly difficult to simply play the game and get the cars yourself. Expensive ones? Just trade for them. No need to actually copy a save where there's nothing left for you to do but hotlap in various 1000+ hp race cars.