My guess is that the mechanic meant to say: if the disk is flexible and has not turned rigid then the disk is good but if the disk is no longer flexible and has turned hard and rigid then the disk needs to be replaced.

If so it sounds like the mechanic is trying to dig up business or he is strongly into preventive maintenance.

My guess is that there is still plenty of life in the current disk and the disk material is flexing enough to do what it is supposed to do... that is take initial stress off the drive train.

It prevents the rear end "clanging" sound once common and possibly still so in Fords and Chevys.

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Earl



1993 190E 2.3
2000 Toyota 4x4 Tundra