The magnets are made that way because they can't get the same
strength out a rigid material in that size for the same price.
You probably won't find a harder replacement that will fit.

I doubt they made them fail intentionally. Those magnets
don't fall apart of their own accord, but hard contact will
break them.

wrote:

It would appear that this (and I guess many now) magnet is made by
taking a ferrite magnetic powder and encasing it with a layer what
looks like chrome. The problem appears to be that the outer shell wore
away somehow or was damaged at some point (more likely they knew it
would die after a couple years and people would just buy entire new
drives) and then we only had loose magnetic particles trying to pull
down the upper platter of the clamp and spin it. That explains the
noise and it's failure to spin up properly (especially at 40x).
....
Does anyone have any idea where I might find a non powdered replacement
or what I should look for?