Remember that problem with Samsung's 840 EVO SSD? You know, the one that slowed read speeds with old data? Samsung issued a fix two weeks ago, and at the time, the company told us the older 840 Series was unaffected by the bug. The thing is, there were reports of slowdowns with the 840 Series in the initial forum threads chronicling problems with the EVO.
We asked Samsung how it determined the 840 Series was immune, and we're still waiting on a response. However, German site ComputerBase has published a statement from the company that seems to acknowledge a problem with the older drive. The Google translation is a little rough, but PC Perspective has a better one that says "accessing specific data with units of SSD 840 could lead to lower reading performance." Samsung is reportedly working on a solution, but there's no associated timeline.
Interestingly, the translated Samsung statement goes on to say, "Due to different technologies the PRO-series (840 PRO and 850 PRO) are not affected." That technology difference is the number of bits per cell: MLC drives like Pros have two bits, while the 840 Series and 840 EVO use three-bit TLC NAND.
Samsung attributed the EVO's woes to a software error related to read-retry, a function used to compensate for changes in cell voltages. Since TLC NAND's higher bit density makes it more sensitive to voltage drift over time, it's not surprising to see the 840 Series suffering a similar fate. Let's hope these issues don't affect the upcoming 850 EVO, which crams three bits into 3D V-NAND cells.