How long is the service life of a USB flash stick? Many store important data on USB sticks, but have no idea how reliable it is. And even fewer people know that there are variations between different types of NAND flash memory sticks according to their lifespan. There are two different kind of NAND flash memory: SLC Single Layer Cell (SLC) and Multi Layer Cell (MLC). They not only vary with respect to their useful life, but also support different read and write speeds.

NAND SLC flash memory stores one 1 bit in every transistor whereas NAND MLC flash packs 2 bits at each memory cell. Hence, with MLC flash one can store more data on a device with the same physical size. Another advantage of MLC is that it is a cheaper than SLC, that is for a given memory size its price is lower.

However, the downside of MLC is that it is slower than SLC. The read/write performance between different types of memory sticks can be huge. Some of the more expensive sticks are almost as fast as hard disks. I’ve seen sticks that read data with 34 MB/s and write with 21 MB/s. If the vendor is silent about the speed it could even be below 1 MB/s.

Another disadvantage of MLC memory is their lower reliability. MLC is usually rated to have about 10,000 write/erase cycles whereas SLC can reach 100,000 to 300,000 cycles. However, this doesn’t necessarily mean that SLC sticks are 30 times more reliable than MLC memory. Modern USB stick controllers use the so-called wear-leveling technique to extend the lifespan. The German computer magazine C’t (20/2007) tried to destroy a USB stick by writing constantly to the same logical address. They gave up after 16,000,000 cycles.

So some USB sticks seem to be quite reliable. However, I wouldn’t rely on those very cheap sticks you can get everywhere now. There is one factor that makes all USB sticks unreliable though, independent of the price or technology: It is their small size. They are easily forgotten or even lost. That is why it is necessary to take some precautions regarding security and backup. In one of my next posts I’ll write about the steps that can be taken to secure a USB stick.