Each of the memory cells can be written to a finite number of times. The drive showing 40% of its useful life remaining has used up the other 60% of the original number of "write events". Eventually the cells will no longer be able to be written to, because the voltage required to write to them is now too high (explained in the manual, and news to me). I do not think anything more can be said about that, other than to keep an eye on it, which is what the program is doing, automatically,
It will be interesting to see, after the program has been in use by more people over more time, just how low some of the numbers get. Thanks for providing your details.