![]() The 840 Evo’s problem was that the calibration algorithm that’s supposed to detect the voltage levels in the cells apparently wasn’t calibrated correctly. 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. As time passes, the state shifts slightly and the drive has to compensate for that shift in order to read the older data. When data is written to triple-level (TLC) NAND, it’s stored at one of eight distinct voltage states. 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. Samsung has released its 840 Evo SSD repair tool, as well as additional information on what causes the problem.
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