The main kinds of portable storage
Portable storage is anything you can carry and connect to different devices. The common types are USB flash drives, memory cards, portable solid-state drives and portable hard drives.
- USB flash drives (also called thumb drives or sticks) are small, plug into a USB port, and are handy for moving files between computers.
- Memory cards, mostly SD and microSD, are used in cameras, drones, handheld game consoles and many other devices. A card reader or an adapter lets a computer read them.
- Portable solid-state drives are small external drives with no moving parts, generally faster than flash sticks and offering larger capacities.
- Portable hard drives use spinning disks. They can offer a lot of capacity for the price, but they are more sensitive to knocks and drops while running.
How flash storage works
USB sticks, memory cards and solid-state drives all use flash memory. Flash stores data by trapping tiny amounts of electric charge in cells, and the pattern of charge represents bits. It keeps its contents without power, which is why you can unplug a stick and find your files later. A solid state drive is built on the same principle, and the short version is that flash has no moving parts and is read electronically.
A controller chip inside the device manages where data is placed. It spreads writes around so that no single group of cells wears out too soon, a technique called wear levelling.
Wear and lifespan
Flash cells can only be rewritten a limited number of times before they become unreliable. For typical home use this limit is rarely reached, but heavy use, such as constantly recording video or running a program from a cheap stick, can wear a device faster. Budget products tend to vary more in quality, and capacity claims on unbranded devices are not always honest, so buy from a reputable seller and test a new drive by copying real files to it and reading them back.
Memory cards come with endurance-oriented versions sold for uses like dashboard cameras and security cameras, which write continuously. A standard card may not be designed for that.
What the speed ratings mean
Speed labels on memory cards can look like a code. Some of the common ones:
- Speed Class (a number inside a letter C, such as Class 10) gives a minimum sustained write speed. Class 10 means at least 10 megabytes per second.
- UHS Speed Class (a U with a number inside) is similar for UHS cards. U1 means at least 10 megabytes per second and U3 at least 30.
- Video Speed Class (V6, V10, V30, V60, V90) states the minimum sustained write speed in megabytes per second, aimed at video recording. V30 means at least 30.
- Application Performance Class (A1 and A2) relates to how well a card handles the small, scattered reads and writes typical of running apps, which matters for phones, tablets and some handheld devices.
Higher is not always necessary. A card for a basic photo camera does not need the speed that a camera recording high-resolution video does. The manufacturer’s recommendation for your device is a good guide.
Beware of “up to” figures printed on packaging. They usually describe best-case read speed, not what you will see when writing many files. Also, the speed you get depends on the whole chain: the card, the reader, the port and the device. A fast card in a slow reader will perform like a slow card. USB drives have a similar situation; the USB version and the port it is plugged into limit how fast data can move, and a device labelled with a fast standard may run slower in an older port.
Capacity and formatting
Drives and cards come pre-formatted with a file system. Many smaller SD cards use FAT32, while larger ones and many USB drives use exFAT, which handles bigger files and is read by most modern computers and cameras. FAT32 has a limit on the size of any single file, roughly 4 gigabytes, which can be a surprise when copying a large video. Formatting erases everything on the drive, so copy anything you need first.
How long data stays readable
Flash memory is not an archive. The stored charge can leak slowly over time, and the rate depends on the device, its wear, and the temperature. Data on an unplugged device is often readable for years, but there is no dependable guarantee, and a single stick in a drawer is not a safe home for irreplaceable files over the long term. Heat, humidity and heavy use can reduce the time further.
Other things that endanger data on portable storage are more mundane than aging:
- removing the device while it is still writing, which can corrupt files or the whole file system;
- loss, theft or being run through the wash;
- static, physical damage or bent connectors;
- fake or faulty devices that report more space than they have, leading to silently lost files.
Practical advice
- Keep important files in at least two places. A portable device works well as one copy but is a risky only copy; a cloud backup or second drive is a common precaution.
- Use the “eject” or “safely remove” option before unplugging.
- Copy photos off a memory card promptly, and avoid filling it right to the brim.
- Replace old devices that begin to show errors, and do not keep using a card that has started to fail.
- Be careful with unknown USB drives. A stick found or handed to you may contain harmful files, so do not plug it into a computer that holds important data.
- If your data is sensitive, consider encrypting the drive so that someone who finds it cannot read the contents. Be aware that if you lose the password, you may lose the data too.
If a device stops being recognized, stop using it and try another port or computer first. Data recovery is sometimes possible but never guaranteed, which is another reason to keep a second copy.
Portable storage is convenient, cheap and fast for moving files around. It works best as a courier for your data, with a separate long-term home for anything you cannot afford to lose.