thats a security torx head, not a standard torx. the tool must have a little hole down the centre. get a set of security bits and they should fit. its so you cant easily undo them, as you're finding out!
the good news is that security bits work on standard heads (but not the other way round) so you only need the one set.
or a cheaper option would be to replace the screw with a standard headed torx, phillips, slot, socket etc screw
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dave
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