What is an SD Card?
SD cards, or Secure Digital cards, are a type of removable flash memory card commonly used in digital cameras, mobile phones, tablets, and other portable devices to store media like photos, videos, and music files. The SD standard was introduced in 1999 by SanDisk, Panasonic and Toshiba as an improvement over MultiMediaCards (MMC).
The SD card format has gone through several evolutions over the years. The original SD card stored up to 2GB of data and used the SPI bus interface. SDHC (Secure Digital High Capacity) launched in 2006 offered up to 32GB capacity. SDXC (Secure Digital eXtended Capacity) in 2009 increased the maximum capacity to 2TB. Speed classes for SD cards like Class 2, Class 4, and Class 10 have also been introduced to indicate the minimum read/write performance.
Today’s SD cards are available in three form factors – standard size, mini size, and micro size. Standard SD is the largest at 32mm x 24mm x 2.1mm. MiniSD is 28mm × 20mm × 1.4 mm. MicroSD is the smallest at 15mm × 11mm × 1mm. A key feature of SD cards is their small size while still offering significant storage capacity, making them ideal for portable devices where space is at a premium.
USB Mass Storage
USB mass storage allows external devices like SD cards, USB flash drives, and external hard drives to be accessed by a computer as if they were traditional disk drives (Wikipedia). This is achieved through the USB mass storage device class, a USB protocol that provides a framework for communicating with storage devices.
When an external storage device is connected via USB, the host computer can mount it and interact with files and folders on the device similar to an internal drive. This allows for easy data transfer between devices. The USB mass storage class uses the standard SCSI command set to communicate instructions like read, write, and delete files with the storage device.
Overall, USB mass storage gives a simple plug-and-play experience for accessing external storage drives. Users don’t need to install special drivers, as support is built into modern operating systems like Windows, Mac OS, and Linux. This makes it a convenient way to add storage capacity or transfer files by connecting portable USB drives.
Using SD Cards as USB Storage
To use an SD card as USB storage and access the files on it from a computer or device with a USB port, you need an SD card reader or adapter that can plug into the USB port. Here are the steps:
1. Get an SD card USB reader or adapter. There are a few options, such as a dedicated SD card reader that connects via USB cable, a USB-C multi-card reader, or a microSD-to-USB adapter. Make sure it fits the type of SD card and computer/device USB port you have (USB-A, USB-C, etc).
2. Insert the SD card into the card reader or adapter. Line up the card so the metal contacts are facing the correct direction.
3. Plug the reader/adapter into an open USB port on the computer/device. For adapters that are USB-C on one end, you can plug it directly into a USB-C port.
4. The computer should detect the SD card and show it as a connected drive with its contents accessible in File Explorer (Windows) or Finder (Mac). The drive letter may vary.
5. You can now access, view, copy and manage the files on the SD card through the USB connection just like any other drive.
This allows transferring files between the SD card and computer without having to remove the card and use a built-in SD slot. It’s convenient for accessing the card and data from devices that don’t have an SD card reader.
Benefits
SD cards offer several key benefits compared to USB flash drives that help explain their popularity:
Portability: SD cards are extremely small and lightweight, with even high-capacity cards being smaller than a fingernail. This makes them easy to carry around and use on the go without taking up much space [1].
Wide Compatibility: SD cards use a standard format supported across countless devices, from DSLR and mirrorless cameras, to smartphones, tablets, laptops, drones, gaming consoles, and more. Their ubiquity makes file transfer seamless [2].
Easy File Transfer: Popping an SD card in and out of devices is quick and convenient for sharing files, vs fumbling with connecting USB cables. SD slots are standard on most cameras, laptops, and other electronics [3].
Limitations
SD cards have some limitations compared to USB flash drives. One main limitation is speed. USB 3.0 flash drives have maximum theoretical transfer speeds of up to 625MB/s, while the fastest SD cards top out around 300MB/s (Skidmore, 2022[1]). So for moving large files quickly, USB drives have a speed advantage.
In terms of storage capacity, SD cards currently max out at 1TB while high-capacity USB drives can store up to 2TB, sometimes more (Lee, 2021[2]). For massive amounts of portable storage, USB drives have larger options available.
These limitations in speed and storage capacity means USB drives tend to be better suited for things like full system backups, storing large media files, running operating systems, or any application requiring fast write speeds.
Compatibility
Many devices support using SD cards as USB storage thanks to USB card readers and adapters. According to Amazon, SD card USB adapters allow you to use SD cards as portable USB storage on devices like laptops and desktop computers. The most compatible devices include:
- Windows PCs
- MacBooks
- Chromebooks
- Android smartphones and tablets
- iPhones and iPads (with USB camera adapters)
Some new Android phones like the Samsung Galaxy S21 and tablets come with built-in SD card slots, allowing you to access the SD card directly as portable storage without any adapters. According to EclipseCat, USB card readers support the major SD card formats including SD, SDHC, SDXC, microSD, and microSDHC.
Speed Comparison
SD cards are rated by speed classes that indicate their minimum guaranteed speeds. Common speed classes are Class 2, Class 4, Class 6, and Class 10, with higher numbers indicating faster minimum write speeds. However, maximum speeds can vary greatly.
The fastest SD card interface is UHS-II, which has maximum theoretical transfer speeds up to 312MB/s (Kingston). However, real-world speeds rarely reach the maximum.
USB flash drives generally have slower speeds than the fastest SD cards. Most USB 2.0 drives provide speeds of around 30-60MB/s for large file transfers. USB 3.0 is faster at around 100-600MB/s, but still slower than high-end UHS-II SD cards (Reddit).
Therefore, using an SD card for USB storage can provide much faster transfer speeds compared to a standard USB flash drive. However, speed depends on the specific SD card model and class. The fastest UHS-II cards offer comparable or better performance than USB 3.0 drives.
Use Cases
SD cards can be a convenient and useful alternative to dedicated USB flash drives in certain situations. The main distinction in when to use an SD card for USB storage versus a dedicated USB drive comes down to the use case and your specific needs.
Some common cases when it may make sense to rely on an SD card for USB storage include:
- When you need expanded storage on a laptop or device with an SD card slot. Using the SD card as extra USB storage can be simpler than carrying around an external hard drive or USB drive.
- For quickly transferring files from devices with SD slots like cameras to a computer. An SD card can make a direct file transfer without cables.
- When you already have an SD card on hand and don’t have access to a USB drive. SD cards are ubiquitous and many of us have them already for devices like cameras.
- When size or portability is paramount. SD cards are tiny and fit in your wallet, unlike bulkier USB drives.
However, for general everyday file storage and transfer, a dedicated USB flash drive is often preferable for reasons like:
- Faster transfer speeds with USB 3.0/3.1 versus most SD cards using USB 2.0.
- More reliably working across different operating systems and devices.
- Not needing an SD card reader for devices without built-in SD slots.
- Less prone to physical damage from repeated inserting/removing.
In summary, SD cards can serve as portable USB storage in a pinch but dedicated USB drives tend to work better for frequent everyday file transfer needs. Consider the use case and your specific needs to decide if relying on an SD card makes sense or if a USB drive would be more suitable.
Security Considerations
When using SD cards as USB storage, it’s important to be mindful of security risks, especially when using public computers. As the CISA notes, “When the USB drive is plugged into another computer, the malware infects that computer” (https://www.cisa.gov/news-events/news/using-caution-usb-drives). Unsecured SD cards and USB drives are vulnerable to malware infections that can spread from computer to computer.
To mitigate risks, experts recommend encrypting SD cards to add a layer of security. Encryption transforms data into unreadable code that requires a password or key to decipher. This protects sensitive files in case the SD card is lost or stolen. Some SD cards even come with built-in encryption features. Third-party encryption software like BitLocker or VeraCrypt can also be used to encrypt SD cards (https://counterespionage.com/usb-memory-security/).
When using public computers, it’s best to avoid saving sensitive files or information directly to the SD card. If files must be transferred, encryption provides vital protection. Users should also scan the SD card with antivirus software before using it again on their own computer.
With proper precautions, SD cards can be safely leveraged as USB storage. Encryption, malware scans, and care when using public machines can help minimize security risks.
Summary
To recap, SD cards can be used as USB mass storage devices to transfer files between devices. This allows you to easily access the files on an SD card by plugging it into a computer’s USB port without needing a card reader. Some key benefits are portability, wide compatibility with devices, and fast transfer speeds. However, there are some limitations to be aware of, like SD cards not having high capacities compared to USB drives, and the file transfer process being slower than reading/writing directly from the SD card slot. Overall, using SD cards as USB storage is a convenient way to access media files and documents when you don’t have a dedicated card reader available. Just be mindful of the capacity and speed tradeoffs.
