Samsung has released its 840 Evo SSD repair tool, as well as additional information on what causes the problem. It turns out that the issue is a bug in how the drive calculates what the voltage level within a cell should be in order to perform a proper data read. When data is written to triple-level (TLC) NAND, it’s stored at one of eight distinct voltage states. As time passes, the state shifts slightly and the drive has to compensate for that shift in order to read the older data. Auto-refreshing the data with a periodic re-write isn’t an option — TLC NAND already has lower durability than other form factors, and a periodic background rewrite would quickly exhaust the number of program/erase (P/E) cycles.