Did you uninstall everything? Support files, instrument packs, etc. If you did then I don't know the answer. I had a similar error and this resolved the issue for me.
Contact Apple for support. You did buy it, didn't you?
Should i also delete the Apple Loops?
You did buy it, didn't you?
just install 6 directly?
Yes. Every file, loop, and/or instrument associated with GarageBand. If you already have music created then of course remove those files first.
Don't go there. Nevertheless, I am assuming this is from a Disc or the Mac App Store. So OP, if not, you're probably going to have to pursue another route altogether with this install.
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Yes.
Do you think it got corrupted when i tried reinstalling from my external HD?
In a nutshell, yes. In my case and in others I have read that is indeed the case. FYI, you should still be able to use Garageband after acknowledging the error. Also, in some cases users have to resort to using Garageband after first dismissing the error because a clean install didn't solve the problem. Good luck!