A memory card carries a cluster of symbols — C10, U3, V30, A2, UHS-II — that look like the same claim repeated. They are not. Some describe minimum sustained write speed, one describes how many small files the card can handle per second, and one is not a speed rating at all: it tells you which bus the card runs on, which sets the ceiling for everything else.
Here is what each symbol means, followed by the device and recording-mode checks to make before buying.
1. First, the format
| Type | Typical use | What caps its speed |
|---|---|---|
| microSD (microSDHC/microSDXC) | Phones, tablets, dash cams, security cameras, drones, action cameras, Switch 1 | UHS-I or UHS-II bus |
| microSD Express | Nintendo Switch 2 (few other devices so far) | PCIe/NVMe |
| SD (SDHC/SDXC) | DSLRs, mirrorless bodies, compacts, some camcorders | UHS-I or UHS-II bus |
| CFexpress Type A | Certain Sony bodies (usually a combo SD/Type A slot) | PCIe/NVMe |
| CFexpress Type B | Higher-end Nikon, Canon and Panasonic bodies | PCIe/NVMe — the fastest of the group |
Check the camera manual before buying. Type A and Type B are physically different and will not fit each other's slots; Type A bodies usually take SD in the same slot.
2. What the symbols actually measure
Speed classes: minimum sustained write
Video Speed Class (V6, V10, V30, V60, V90) and UHS Speed Class (U1, U3) both state a minimum sustained write speed, and the number is the figure in MB/s. V30 means at least 30MB/s, V90 at least 90MB/s; U1 corresponds to 10MB/s and U3 to 30MB/s. Video recording fails when writes cannot keep up, which is why camera and drone spec sheets say "V30 or above required".
Application Performance Class A1/A2: small files
A1 and A2 are not about video. They exist for phones and tablets that run apps directly from the card. The SD Association defines them as:
- A1 — at least 1500 random read IOPS, 500 random write IOPS, and 10MB/s sustained sequential write
- A2 — at least 4000 random read IOPS, 2000 random write IOPS, the same 10MB/s sustained write, plus mandatory support for command queuing, caching and self-maintenance
A2 describes application-oriented random access under supported host conditions. It does not establish a benefit for every CCTV workload, replace a video's required V/U class, or certify endurance. Confirm the host's A2 support and intended workload before choosing on that basis.
UHS-I and UHS-II: the actual ceiling
UHS is not a class, it is the bus the card uses, and it caps everything:
- UHS-I — up to 104MB/s in SDR104 mode
- UHS-II — up to 312MB/s half duplex (there is a second row of pins on the back of the card)
- SD Express — runs on PCIe; up to 985MB/s on a Gen 3 single lane
Keep the card's sustained-write class separate from its bus ceiling. Match the card's class and supported interface to the camera manual; a theoretical interface ceiling does not itself guarantee sustained recording. A compatible UHS-II card used through a UHS-I host cannot use the UHS-II transfer path.
3. Which card for which device
Phones and tablets
A1 or A2 with U1/U3 is plenty; capacity matters more than speed. The Kodak Ultra Performance microSD (U3 V30 A1) is this category.
Nintendo Switch
Switch 1 takes ordinary microSD. Switch 2 only accepts microSD Express — ordinary cards cannot expand Switch 2 software storage. microSD Express can hold software, screenshots and videos; save data remains in system memory even when an Express card is installed. It is the single most common mis-purchase right now, so we gave it its own article: which memory card the Switch 2 needs.
Dash cams and security cameras
Continuous and loop recording impose repeated writes; event recording has a different workload. Confirm card format, capacity and speed in the exact camera manual, and consider an endurance or surveillance-rated card such as the Kodak Endurance microSD (U3 V30 A2), Reolink surveillance card or SpotCam Extreme 128GB. For how cards fit into a camera setup, see our home CCTV guide.
Action cameras, drones, 4K video
Use the exact device's required card format and sustained-write class for the chosen recording mode. V30, V60 or a faster class may be required, but 4K, frame rate or bit depth alone does not establish the minimum.
Burst shooting, 8K, high bitrates
This is where UHS-II earns its price: the Kodak Ultra Pro Performance SD UHS-II V90, or in microSD form the Ultra Pro Performance UHS-II V60. The difference shows up as how quickly the buffer clears after a burst.
Professional bodies
CFexpress: Type A for supported Sony models, Type B for supported Nikon, Canon and Panasonic models; match the exact slot and recording mode, not just the brand. Both run PCIe/NVMe and sit in a different performance class from SD.
4. Sizing the card: work from bitrate
How much space footage takes is set by its bitrate, and the arithmetic is simple: bitrate in Mbps ÷ 8 = MB per second, then multiply by 3600 for an hour.
- 4K at 100 Mbps ≈ 45GB per hour → roughly 2.8 hours on a 128GB card
- A security camera at 4 Mbps ≈ 1.8GB per hour → roughly 3 days of continuous recording on 128GB, far longer with motion detection
- Phone photos and clips: the constraint is usually capacity, not speed — 128GB is a sensible floor
Allow for formatting overhead and space needed by the device; these bitrate estimates assume a constant rate and no extra files. For loop recording, follow the camera's overwrite and maintenance instructions rather than reserving a fixed percentage as a lifespan guarantee.
5. Avoiding counterfeit cards
The classic fake reprograms the controller so the computer reports 512GB when the flash inside is 64GB. Write past the real capacity and it quietly overwrites earlier files — you find out after a full day of shooting. What actually helps:
- Buy licensed stock with a warranty trail. A large card at an implausible price is not a bargain.
- For a new empty card, use a capacity test tool such as H2testw on Windows or F3 on Mac/Linux and follow its instructions. Back up existing files first; a full write-and-verify test can fill or overwrite the card.
- Look at the printing: genuine cards have crisp text and laser marks, with model and capacity square on the label.
- Check that the printed class, interface and model agree with the manufacturer's official listing. Inconsistent markings need verification; appearance alone does not establish that the capacity or speed claim is genuine.
6. Readers and backups: the whole chain has to be fast
A fast card does not mean fast transfers. A UHS-II card in a UHS-I reader runs at UHS-I speed, and the cable and port can bottleneck it too. Cards are also the wrong place to keep photos long term — copy them to an SSD or external drive, and keep two copies. The Kodak X200 portable SSD and the ASUS M.2 SSD enclosures exist for exactly that. Everything is in memory cards & SSDs.
7. As a gift
A compatible memory card can be a useful addition to a camera, dash cam or drone gift. Confirm the recipient's exact device and recording mode before choosing. Switch 2 can use system memory; add microSD Express if more software storage is needed. More pairings in our Christmas gift guide.
FAQ
Should I format a new card?
Follow the device manual's setup and formatting instructions, preferably using its own format option where provided. Formatting erases files, so back up anything you need first. Do not assume a computer's quick format creates the file system required by every device.
Why is my real-world speed below the number on the card?
The V and U ratings are minimum sustained write figures, not peak speeds — and peak speed depends on the reader, the USB port and the computer. Thousands of small photo files will also always transfer slower than one large video file.
When should a card be replaced?
Cards are consumables. In dash cams and security cameras, check footage regularly according to the device's maintenance guidance, including after recording errors or changes. Recording gaps, files that will not open, or a device that keeps asking you to reformat are all signs to replace it now rather than later.
What is the little switch on a full-size SD card?
A write-protect lock. If it has been flipped, the device reports the card as protected or write-failed — the switch may explain the error, but persistent failures still need checking.
Ordering and warranty
Kodak, Reolink and SpotCam cards and SSDs are Hong Kong licensed stock with brand warranty. Confirm current stock, delivery timing and collection arrangements before ordering; our Kwun Tong location is for stock and collection. Not sure which card your device takes — WhatsApp +852 9847 9793 with the model.
