Also known as: Secure Digital, microSD
An SD card (Secure Digital) is a small removable storage card built around flash memory, widely used in cameras, phones, and small computers.1
Overview
The standard defines several physical sizes — full-size SD, miniSD, and the tiny microSD — and capacity tiers that grew over time: SDHC, SDXC, and SDUC. A small controller inside the card handles wear leveling, bad-block remapping, and error correction, then presents a simple block device, so the host just sees storage formatted with a file system (usually FAT or exFAT).
Ratings on the card describe two different things. The capacity tier (SDHC, SDXC) sets how large the card can be and which addressing the host must support. The speed class (Class 10, UHS Speed Class, and the video V-classes) rates the minimum sustained write performance — the guaranteed floor under continuous recording, which matters far more than the flashy peak read numbers on the label.
Speed classes
The class marking states the slowest write the card is allowed to sustain, so a recorder never outruns it:
| Class mark | Min sustained write | Typical use |
|---|---|---|
| Class 10 / U1 | 10 MB/s | HD video, general |
| U3 / V30 | 30 MB/s | 4K video capture |
| V60 | 60 MB/s | High-bitrate 4K/8K |
| V90 | 90 MB/s | Professional 8K |
Where it fits
The SD card is the default boot and storage medium for many single-board computers, including the Raspberry Pi, which boots from a microSD card by default. It is cheap and removable, but cards vary widely in quality and endurance, and a low-end card can wear out under constant logging. A GopherTrunk capture node can run from an SD card — a well-rated card easily keeps up with decoded-call logs — though for heavy continuous IQ writes an eMMC module or an SSD lasts longer.