Wirebook › Glossary › SLC cache
What is an SLC cache (why does a USB copy slow down)?
In one sentence: A small, fast part of a USB stick's or SSD's memory that new data is written to first, so copies start quickly and slow down once it fills.
A little more
Flash memory stores data by packing several bits into each tiny cell, which is cheap but slow to write. Many USB sticks and SSDs set aside part of their memory to work in a quicker one-bit-per-cell mode (single-level cell, SLC). New data lands there first at full speed, and the drive moves it into the dense, slower storage later, when it's idle.
On a big copy the fast area fills up, and from then on the drive has to write straight into slow storage while also tidying the cache, so the speed can drop to a quarter or less. That's the race-then-crawl you see on large files. It's normal, not a fault. Leaving a drive plugged in and idle lets it empty the cache, and drives with a bigger cache or faster memory hold their speed for longer.
See it in action
Do it for real
Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.