I have an old 530 series 120 SSD, and in Intel SSD Toolbox it shows ~5% life remaining. So what will happen when it reaches 0? Will it stop writing, or block, or else?
AFAIK, Intel is basing this life remaining assessment purely upon the current S.M.A.R.T. attribute data (see here: https://en.wikipedia.org/wiki/S.M.A.R.T. for an introduction to S.M.A.R.T.). Their "0% lifetime remaining" point is the point where the drive is itself reporting its impending failure. What you do at this point is well stated by the recommended text in the S.M.A.R.T. specification: "WARNING: Immediately back-up your data and replace your [hard disk] drive. A failure may be imminent."
