A flash drive, or USB key, is a removable storage device that relies on NAND flash memory to retain data without electrical power. Unlike a mechanical hard drive, this component has no moving parts, making it silent and shock-resistant. The term “USB” refers to the connection interface (Universal Serial Bus), while “flash drive” refers to the storage technology used: both terms describe the same object.
NAND Cells and Electron Trapping: What Happens Inside the Chip
Storage in a flash drive is based on NAND memory cells organized in a grid. Each cell contains a floating gate transistor, a conductive layer isolated between two oxide layers. To write data, an electric current forces electrons to tunnel through the oxide layer and settle in the floating gate.
Once trapped, these electrons remain in place even without current, making the storage non-volatile. To erase, a reverse voltage pushes the electrons out of the gate. This write and erase mechanism gradually wears out the oxide layer, which explains that each NAND cell has a finite number of write cycles.
To delve deeper into the operation of a flash drive and USB, it is also important to distinguish between types of cells. SLC (Single Level Cell) cells store one bit per cell and offer the best endurance. MLC, TLC, and QLC cells stack two, three, or four bits per cell, respectively, increasing capacity but reducing longevity and write speed. The vast majority of consumer USB keys use TLC or QLC chips.
USB Controller and Transfer Protocol: From Port to Memory
The flash drive is not just its NAND chip. An integrated controller manages all operations between the computer and the memory. This microprocessor translates commands from the operating system into instructions understandable by the NAND cells: reading, writing, block erasing.
The controller also ensures “wear leveling,” an algorithm that distributes writes across all cells to prevent a single block from wearing out prematurely. Without this function, the most frequently accessed sectors (file allocation table, for example) would reach their cycle limit long before the rest of the chip.
USB-C Does Not Guarantee Transfer Speed
A common source of confusion: USB-C refers to the shape of the connector, not the speed. Two keys with identical USB-C connectors can operate at radically different speeds depending on the version of the protocol they support. To estimate the actual transfer speed, three elements must be checked: the specification of the key, that of the computer port, and, if applicable, that of the cable or adapter used.
Since April 2026, the European directive on the universal charger also applies to laptops, which generalizes the presence of USB-C ports. This requirement concerns charging, but it has the collateral effect of multiplying physically identical ports with very varied transfer capabilities.
File System and Device Compatibility
When an operating system recognizes a USB key, it reads its partition table and its file system to organize access to the data. The choice of format directly affects compatibility with different devices.
- FAT32 is the most universal format: readable by Windows, macOS, Linux, game consoles, car radios, and televisions. Its main limitation is a ceiling per individual file, making it unsuitable for large files like disk images or long high-definition videos.
- exFAT removes this size constraint while remaining compatible with most modern systems. It is the recommended format for transferring large files between computers.
- NTFS offers advanced features (journaling, permissions) but is primarily native to Windows. macOS reads it without third-party software but does not write to it by default.
Formatting a key in a file system unsuitable for the target device is the most common cause of “unrecognized key.” Before copying documents to a key intended for a television or car radio, checking the expected format in the device’s documentation avoids unnecessary reformatting.
USB Key in Daily Life: Concrete Limitations to Know
A flash drive is reliable for transporting files but is not designed as a primary backup solution. NAND memory degrades over time, even without use, because the trapped electrons in the floating gate eventually escape. A key left in a drawer for several years may lose data.
Removing without software eject remains a real risk. When the operating system writes to the key, it uses a cache in RAM. Unplugging the key during this phase can corrupt the file system and make files unreadable. On Windows, the “Safely Remove Hardware” function forces the cache to flush before physical disconnection.
Transferring Large Files
The perceived copy speed depends on three combined factors: the transfer rate of the USB protocol supported by the key, that of the computer port, and the actual write speed of the NAND chip. Many keys advertise high read speeds, but their write speed is significantly lower.
Copying a folder containing thousands of small files also takes much longer than copying a single file of equivalent size because each file requires a distinct operation on the allocation table.
The flash drive remains an effective daily data transport tool as long as its physical constraints are understood. The choice of file system, checking the actually active USB protocol, and software ejection before removal are the three reflexes that prevent most data loss on USB keys.
