NAND-Flash-Speicher

Whether SSD, USB stick, memory card or smartphone: today almost everywhere the same type of memory is found – NAND flash. It is fast, robust and energy-efficient. For secure erasure, however, it has peculiarities that you should know. This entry explains the structure and the consequences for IT decommissioning.

Structure and how it works

Flash memory was developed in Japan in the 1980s. The NAND design is named after the wiring of the cells, which resembles a logical NAND gate. It allows high storage density at low cost. That is why NAND has become established for mass storage.

The cells are organized into pages and blocks. Reading and writing happen page by page, erasing only for entire blocks. A written page cannot simply be overwritten. The controller therefore writes changed data to a new location and marks the old one as invalid. A mapping table, the Flash Translation Layer, keeps track of where which data is physically located.

Cell type	Bits per cell	Typical characteristic
SLC (Single-Level Cell)	1	Very durable, expensive, mainly in specialized applications
MLC (Multi-Level Cell)	2	Good compromise between durability and price
TLC (Triple-Level Cell)	3	Widely used in SSDs today
QLC (Quad-Level Cell)	4	High capacity, lower write endurance

Modern chips stack many cell layers on top of each other (3D NAND). Each cell can only withstand a limited number of write and erase cycles. So that they age evenly, the controller distributes the write operations (Wear leveling) and keeps spare blocks in reserve (Over-provisioning).

Where NAND flash is found

Typical places of use are SSDs in laptops and servers, USB sticks and memory cards, smartphones and tablets, as well as permanently soldered storage such as eMMC in inexpensive laptops, thin clients and embedded devices. NAND flash is also built into printers, network devices and vehicles.

What to note when retiring devices

• Overwriting is not always enough: Because the controller redistributes data, old copies may lie in spare blocks that the operating system does not see.
• Suitable erasure methods: Commands that act within the drive itself are secure, such as Sanitize or cryptographic erasure, combined with a check of the result.
• Destruction only if fine enough: Flash chips are small and store a lot. DIN 66399 assigns them to material class E, with considerably smaller particles than for hard drives.
• Permanently built-in storage: eMMC or soldered SSDs cannot be removed. They are erased in the device or destroyed with the device.

Which methods are suitable for SSDs is shown by our guide Securely erasing hard drives and SSDs.

Relation to Second IT

Second IT erases SSDs, HDDs and mobile devices with Blancco and documents every erasure in a digitally signed erasure certificate per device. Data carriers that cannot be erased securely are destroyed in-house according to DIN 66399. At goods receipt, all devices are checked for storage media – including forgotten USB sticks and memory cards. More about certified data deletion.

Frequently asked questions about NAND flash

What is the difference between flash memory and a hard drive?

A hard drive stores data magnetically on rotating platters. Flash memory stores it electrically in semiconductor cells, without moving parts. This makes it faster and more robust, but changes the way data has to be erased.

Does flash memory lose its data without power?

No, NAND flash is non-volatile. The charge is retained for a long time. Over very long periods without power, however, it can fade, especially in heavily worn cells. This is not suitable as an erasure method.

Can data be recovered from an erased USB stick?

After a simple deletion or quick format, often yes. Only a suitable erasure method with verification or destruction according to DIN 66399 makes the data permanently unreadable.

What is the difference between SLC, MLC, TLC and QLC?

The abbreviations indicate the bits per cell: SLC stores 1, MLC 2, TLC 3 and QLC 4. SLC is very durable but expensive. TLC is widespread in SSDs. QLC offers high capacity with lower write endurance.

Why can flash memory not simply be overwritten?

Flash is written page by page but erased only block by block. The controller writes changed data to a new location and marks the old one as invalid. Old copies may lie in spare blocks that the operating system does not see.