Alright, some more information to go on.
As you can see in the above image, you can see how TLC NAND Flash works. TLC which stands for Triple Level Cell and is named that because it stores three bits or three binary states per cell. 000, 001, 101, 011, 100, 101, 110, and 111
Each binary state requires a different voltage state to represent the binary state in the NAND flash cell, specifically eight different states. This is much different from traditional MLC NAND in which it only has to hold four different binary states.
Notice how you have two voltage states, the 010 and 011 states. They are closer together because the voltage state drifted. When we read the cell and we get a cell voltage back we have an issue. Does that cell contain a 010 or 011?
So now, we have to read the flash cell multiple times to come to a general consensus on what that cell is storing. If a majority of reads come back with a voltage state that represents 011 then it must be 011.
Therein lies the problem. Because of voltage drift in the NAND Flash Cell a Flash Cell must be read multiple times because we need to figure out what that cell is storing because of voltage drift.
Because both the 840 and 840 EVO use much smaller NAND Flash Cells, 21nm and 19nm respectively. Because the flash cell is so small there’s not a lot of room to store a lot of electrons thus any change in voltage state caused by an electron or two escaping the flash cell can cause the voltage drift to be very detrimental to reading the binary state back from the NAND flash cell.
Samsung has gone back to 40nm with their new 3D NAND technology which should in theory solve the issues with voltage drift but like the 840 EVO, the 850 EVO is also TLC or Triple Level Cell and thus open once again to voltage drift inside the cell effecting read speeds because of the many various forms of voltage states needed to represent eight voltage states for eight binary states.
With that being said, Samsung should be forced to do a massive world wide recall of the 840 and 840 EVO because the voltage drifts can become a bigger issue with reading data back as the NAND flash cells become older and more writes have been done to the flash cell. More voltage drifts means more time needed to read data and quite possibly lead to mass data corruption due to the inability to read data back from older flash cells.
But, Samsung won’t do this. This is why I call for a boycott on Samsung SSDs until they do right for the owners of the 840 and 840 EVO. They are playing with people’s data integrity here.