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Rutto said: why can't I fix it in any way ?

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Create a new database (just for testing) and see if you can successfully import just that one table into it.
1) If you can't (and we know the REPAIR TABLE command hasn't worked), then it may be seriously damaged beyond repair. It's difficult to know how fixable it is without actually taking a look at it.
2) If you can import it, then this means that you'll have to DROP the current w3t_Posts table from your main database first *before* importing the backup.