NAND flash is a type of non-volatile memory that keeps data without power by trapping electrical charge in transistor cells. It is the storage technology inside SSDs, USB drives, microSD cards, and phones.
Why it matters
NAND is organised into pages (the smallest unit that can be written, often a few kilobytes) grouped into blocks (the smallest unit that can be erased). A page cannot be overwritten in place; the block must be erased first. A controller hides this with wear levelling, garbage collection, and a flash translation layer. Each erase wears the cells, so every NAND device has a limited write endurance.
Modern chips stack cells in layers, called 3D NAND, to raise capacity without shrinking cells further. Cell type (SLC, MLC, TLC, QLC) sets the balance of cost, speed, and endurance.
Example
If you change one 4 KB file in the middle of a full block, the controller writes the updated data to a fresh page elsewhere and marks the old one stale. Later it copies still-valid pages out of the block and erases it. The extra internal writing is called write amplification.
Common mistakes
- Confusing NAND with DRAM. DRAM is volatile and much faster.
- Filling an SSD completely, which reduces performance and increases wear.
- Assuming a flash drive is a safe long-term archive; unpowered cells can lose charge over years.
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