Windows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallCrashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteiTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
The symbols on a microSD card describe different performance characteristics, not one single speed. C2–C10, U1/U3 and V6–V90 specify minimum sustained sequential performance, which matters when a device writes a continuous stream such as video. A1 and A2 describe application-oriented random and sequential access. UHS-I, UHS-II and UHS-III identify the card’s bus interface, or the communication link between card and host.
Read the device manual first, then match its required capacity, class family and interface. A larger number or a newer-looking symbol does not automatically satisfy every requirement.
What the symbols mean at a glance
These marks answer separate questions. The C, U and V families are minimum sequential-performance tiers; A1/A2 concern app workloads; and UHS-I/II/III identify interface generations. The figures below are standard definitions from the SD Association, not guaranteed everyday transfer speeds.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
| Mark | What it describes | Minimum sequential tier defined by the standard |
|---|---|---|
| C2, C4, C6, C10 | Speed Class | The number in MB/s; C10 is 10 MB/s |
| U1, U3 | UHS Speed Class | U1 is 10 MB/s; U3 is 30 MB/s when the supported UHS class and interface are in use |
| V6, V10, V30, V60, V90 | Video Speed Class | The number in MB/s; V30 is 30 MB/s, V60 is 60 MB/s and V90 is 90 MB/s |
| A1, A2 | Application Performance Class | Separate random-read, random-write and sequential requirements for running apps |
| UHS-I, UHS-II, UHS-III | Bus-interface generation | Interface ceilings of 104 MB/s, 324 MB/s and 624 MB/s respectively, with a compatible host |
SD Association guidance says Speed Class marks assure minimum sequential access performance under specified standard conditions. They do not mean the card always reads or writes at exactly that rate, and they do not state the card’s maximum read speed. See the SD Association Speed Class explanation.
#1 Best Overall
- Compatible with Nintendo-Switch (NOT Nintendo-Switch 2)
- Expand your storage in a flash: ideal for Android smartphones and tablets, Chromebooks, and Windows laptops.
- Increase your TV show, movie, and Full HD video[4] recording collections dramatically with up to a massive 1.5TB[1].
- Transfer files fast with up to 150MB/s[2] read speeds and SanDisk MobileMate USB micro 3.0 microSD card reader[6].
- Load apps faster with A1-rated performance[3].
Speed Class: the C symbol
The original Speed Class symbol is a C surrounding a number: C2, C4, C6 or C10. The number is the defined minimum sequential performance tier in megabytes per second. For example, C10 represents 10 MB/s.
“Sequential” means data is written or read in a largely continuous stream. That is the pattern produced by video recording, audio capture and other operations that continuously append data. The rating is a floor under the standard’s test conditions, not a promise that a particular card will sustain that rate in every device, temperature or workload.
Many newer cards use U or V marks instead of, or in addition to, C marks. If a manual specifically asks for C10, select a card that meets that class-family requirement or a higher grade accepted by the host; do not assume that every unrelated symbol is interchangeable simply because its number looks similar.
UHS Speed Class: U1 and U3
UHS Speed Class uses a U-shaped symbol containing 1 or 3:
- U1 indicates a 10 MB/s minimum sequential tier.
- U3 indicates a 30 MB/s minimum sequential tier.
The U rating applies when the card and host use a supported UHS interface and class combination. A card can carry a U3 mark yet perform differently when placed in a host that does not support the relevant UHS mode. Check the device’s specification rather than reading U3 as an unconditional 30 MB/s transfer guarantee.
Rank #2
- EXPAND YOUR STORAGE. Insert your card to add massive storage up to 1.5TB[1] to your Android smartphones and tablets, digital cameras, and laptops.
- SPACE FOR MORE. With expansive capacities up to 1.5TB[1], capture and store hours of Full HD video[4], movies, music, games, photos, and podcasts.
- MOVE FILES FAST. Use your card with the SANDISK QuickFlow microSD UHS-I Card USB-A Reader[6] to achieve up to 195MB/s[2] read speeds [128GB-1.5TB models] and offload your content fast.
- LOAD APPS IN A SNAP. Rated A1[3], the SANDISK Ultra microSD card is optimized for faster app launch and overall app performance.
- EASY CONTENT MANAGEMENT. Easily back up, organize, and transfer your photos and videos with the SANDISK Memory Zone desktop or Android mobile app[5].
Is U3 faster than U1?
As a class threshold, yes: U3’s defined minimum sequential tier is 30 MB/s, compared with 10 MB/s for U1. That comparison is about the standard’s minimum sustained sequential requirement. It is not a comparison of maximum read speed, burst performance or app responsiveness.
Video Speed Class: V6 through V90
Video Speed Class is shown as a V followed by a number: V6, V10, V30, V60 or V90. The number indicates the minimum sequential tier in MB/s. The commonly used thresholds are:
| Video mark | Minimum sequential tier |
|---|---|
| V6 | 6 MB/s |
| V10 | 10 MB/s |
| V30 | 30 MB/s |
| V60 | 60 MB/s |
| V90 | 90 MB/s |
Video modes can create demanding, continuous writes, so a camera may specify a V class. The appropriate mark depends on the recording mode, frame rate and other implementation details, not resolution alone. The SD Association’s Video Speed Class guidance gives broad examples, but the camera or recorder manual controls for a specific model.
Is V30 the same as U3?
Both V30 and U3 define a 30 MB/s minimum sequential tier, so they are equivalent only in that narrow numeric sense. They belong to different class families and are intended to communicate different compatibility contexts. If a host explicitly requires V30, use a card meeting that requirement; do not substitute a U3 mark without checking the manual. Conversely, a U3 requirement should be matched according to the host’s stated class family.
A1 and A2: application performance, not another video class
A1 and A2 are Application Performance Class marks. They address the random and sequential access patterns involved when an operating system installs or runs apps from the card. Random access involves many small, scattered operations, unlike the long, continuous writes measured by video-oriented classes.
Rank #3
- CAPTURE LARGER THAN LIFE. Unlock startling 5K[3] point-of-view and pristine high-res stills with video speed class ratings of U3 and V30[4].
- SPEED BARRIERS SHATTERED. Save precious moments with rapid read speeds up to 245MB/s[2] and write speeds up to 170MB/s[2] [256GB-1TB capacities[1]].
- MAXIMIZE WITH MASSIVE CAPACITY. Record longer and store more with up to 2TB[1] of storage.
- DURABILITY YOU CAN COUNT ON. SANDISK microSD cards are temperature proof, humidity proof, waterproof, shock proof, drop proof, magnet proof, X-ray proof and wear-out proof[7].
- EASY CONTENT MANAGEMENT. Easily back up, organize, and transfer your photos and videos with the SANDISK Memory Zone desktop or Android mobile app[10].
The two marks use separate combinations of random-read, random-write and sequential-performance requirements defined by the SD Association. A2 is the higher application-performance tier, but the benefit appears only when the host and workload support the features and access patterns covered by the standard. A1/A2 do not replace capacity, file-system or interface requirements, and they do not tell you a card’s maximum video-write or read speed. See the Application Performance Class guide.
Free tools Windows power users keep installed
One-click scans. No signup required.
When should you care about A1 or A2?
Consider these marks when a phone, tablet, handheld computer or other device is designed to move applications to removable storage and its manual supports that arrangement. If the card is used mainly for photographs, video files or backups, the device’s required C, U or V class is usually the more relevant performance instruction.
UHS-I, UHS-II and UHS-III: the bus interface
UHS in a bus label means Ultra High Speed, the interface generation used for communication between the card and a compatible host. It is not the same thing as U1 or U3. The SD Association FAQ lists these maximum interface rates:
| Interface | Maximum interface rate listed by the SD Association |
|---|---|
| UHS-I | 104 MB/s |
| UHS-II | 324 MB/s |
| UHS-III | 624 MB/s |
Those are interface ceilings, not a promise that every card reaches the figure. The host must support the same interface generation, and the card’s controller, flash memory, file system and workload also affect actual results. A UHS-II card in a UHS-I-only device cannot make that device communicate at UHS-II rates; it operates according to the host’s supported mode. The SD Association FAQ describes the interface-rate figures and backward-compatibility behavior.
How the markings work together
A single card can show several marks because each one reports a different dimension. For example, a hypothetical card labeled U3, V30, A2, UHS-I would indicate:
The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Rank #4
- Compatible with Nintendo-Switch (NOT Nintendo-Switch 2)
- Save time with card offload speeds of up to 190MB/s powered by SanDisk QuickFlow Technology (Up to 190MB/s read speeds, engineered with proprietary technology to reach speeds beyond UHS-I 104MB/s, requires compatible devices capable of reaching such speeds. Based on internal testing; performance may be lower depending upon host device interface, usage conditions and other factors. 1MB=1,000,000 bytes. SanDisk QuickFlow Technology is only available for 64GB, 128GB, 256GB, 400GB, 512GB, and 1TB capacities. 1GB=1,000,000,000 bytes and 1TB=1,000,000,000,000 bytes. Actual user storage less.)
- Pair with the SanDisk Professional PRO-READER SD and microSD to achieve maximum speeds (sold separately)
- Up to 90MB/s write speeds for fast shooting (Based on internal testing; performance may be lower depending upon host device interface, usage conditions and other factors. 1MB=1,000,000 bytes.)
- 4K and 5K UHD-ready with UHS Speed Class 3 (U3) and Video Speed Class 30 (V30) (Compatible device required. Full HD (1920x1080), 4K UHD (3840 x 2160), and 5K UHD (5120 X 2880) support may vary based upon host device, file attributes and other factors. See HD page on SanDisk site. UHS Speed Class 3 (U3) designates a performance option designed to support real-time video recording with UHS-enabled host devices. Video Speed Class 30 (V30), sustained video capture rate of 30MB/s, designates a performance option designed to support real-time video recording with UHS-enabled host devices. See the SD Association’s official website.)
- U3: a 30 MB/s minimum sequential tier under the supported UHS class conditions.
- V30: a 30 MB/s minimum sequential tier for the Video Speed Class family.
- A2: an application-performance tier for supported app workloads.
- UHS-I: the card’s bus-interface generation, with a 104 MB/s maximum interface rate listed by the SD Association.
These labels do not say that the card always transfers at 104 MB/s, that it has a 104 MB/s write speed, or that every camera can record every V30 mode. They also do not override the host’s capacity or compatibility limits.
Why equal numbers do not make every mark interchangeable
U1, C10 and V10 all correspond to a 10 MB/s class threshold, while U3 and V30 both correspond to 30 MB/s. Nevertheless, a host may check a particular class family or rely on capabilities associated with that family. Match the family named in the manual and meet or exceed its grade. Do not select by number alone.
How to choose a microSD card for a device
- Read the device manual. Record the supported card type, capacity range, required class family, recording mode and bus interface. The manual is the controlling source for a particular phone, camera, drone, console or recorder.
- Match capacity. A card with the right speed mark can still be unusable if its capacity is outside the host’s supported range or if the device requires a particular microSDHC or microSDXC type.
- Match the class family and meet or exceed the grade. If the manual calls for V30, choose V30 or a higher accepted V class. If it specifies U3, follow that UHS Speed Class requirement. The SD Association’s matching guidance explains that a host calling for a class can use the same or a higher grade in that class family; equal numbers from different families are not automatically substitutes. See its host-and-card matching guidance.
- For video, use the recording-mode requirement. Check resolution together with frame rate, bit rate or capture mode as stated by the manufacturer. A camera’s 4K mode may have a different requirement from its lower-frame-rate or lower-bit-rate modes.
- For apps, check A-class support. Choose A1 or A2 only when the device actually runs apps from removable storage and supports the relevant application-performance features.
- Check the interface on both ends. A UHS-II or UHS-III card cannot upgrade a host that supports only an earlier interface. Conversely, a high interface ceiling does not by itself satisfy a sequential class requirement.
- Separate class ratings from advertised maximum speeds. Treat a manufacturer’s maximum read or write figure as a separate specification, with its own test conditions. Do not replace a manual’s class requirement with a marketing maximum.
Common mistakes and how to fix them
Confusing UHS-I with U1
UHS-I is an interface generation; U1 is a speed-class mark. They can appear together, but they answer different questions. Recheck the manual for both the required interface and the required class.
Assuming a high read speed guarantees recording
Sequential class marks concern a defined minimum performance tier, while a package may advertise a much higher maximum read speed. Video recording depends on sustained write behavior and the host’s implementation. Use the camera’s stated V, U or C requirement.
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteChoosing from resolution alone
“4K” or “8K” does not uniquely identify the necessary card. Frame rate, codec, bit rate, multi-file recording and the device’s firmware can change the requirement. Consult the recording-mode table in the manual.
Best Value
- Up To 48MB/s Read Speed
- 10-year warranty
- Easily Back Up Files With "SanDisk Memory Zone" App
- SD adapter included for compatibility with digital cameras
- The 32GB SanDisk Ultra microSDHC UHS-I Memory Card works with any device that has a microSDHC card slot
Putting a fast card in a slow host
The card cannot create a faster bus in the device. If transfers or recording remain below the card’s advertised capability, verify the host interface, adapter, firmware and workload before blaming the card.
Using an unsupported capacity
Capacity and speed are independent checks. A device may reject a card that exceeds its supported capacity even when every speed symbol looks appropriate. Stay within the manufacturer’s stated range.
Transferring files with a card reader
A USB microSD reader is relevant when a computer lacks a suitable card slot or its built-in slot uses an earlier standard. The reader, USB connection and computer interface become part of the transfer path, so they can limit copy speeds. A reader does not change the card’s class rating or make an incompatible host support a newer UHS generation. The SD Association discusses this accessory scenario in its FAQ.
Quick wins for a faster PC:
Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →What the ratings do not tell you
- They do not provide a universal maximum read or write speed.
- They do not guarantee a particular result in every host, temperature, file system or workload.
- They do not establish endurance, warranty, counterfeit status or data-retention behavior.
- They do not override a device’s capacity, card-type or firmware restrictions.
- They do not mean that a newer interface will run at its ceiling in an older host.
Use the marks as compatibility and minimum-performance signals, then verify the device-specific requirements and the manufacturer’s separate maximum-speed specifications.
Why Video Speed Class was introduced
The SD Association announced Video Speed Class on February 25, 2016, as a way for device and card makers to address higher-resolution and multi-file video workloads. In that announcement, then-association president Brian Kumagai said: “With Video Speed Class, SD device and memory card manufacturers can offer the highest-quality video recording to consumers and professionals that keep pace with the awesome video resolutions not only offered today, but anticipated tomorrow.” The statement describes the standard’s intended role at the time; it is not a current product recommendation. The announcement is available as a PDF from the SD Association.
A practical decision rule
Start with the manual, not the largest number printed on a package. Match the supported capacity, then match the named C, U or V family and grade for the workload. Add A1/A2 to the decision only if apps run from the card. Finally, confirm that the card and host share a supported UHS interface. This sequence prevents the most common error: treating several independent symbols as if they were one universal speed scale.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
