Flash storage is convenient, cheap, and the worst choice for anything you need to keep. The USB stick with your photos on it, sitting in a drawer since 2019, is a bet you probably don't realize you've placed.
Here's how flash actually works, why it fails, and what to use instead.
How flash memory stores data
Flash stores data as electrical charge trapped in tiny cells. Each cell holds a charge that represents one or more bits.
Two consequences follow, and they explain everything else:
Writing wears the cells out. Each write cycle degrades the insulating layer slightly. Cells have a finite number of write cycles before they can no longer hold charge reliably.
The charge leaks over time. Even with no writes at all, stored charge dissipates slowly. This is the crucial point for archival use: flash memory left unpowered gradually loses data.
How slowly depends on the cell type, the age of the device, and — significantly — temperature. Warmer storage accelerates leakage.
Why flash is bad for archives
That second property is the one nobody expects.
A hard drive left in a drawer for a decade will usually still read, because the data is stored magnetically and magnetism is stable. A flash drive left in a drawer for a decade may not, because the charge has drained.
Manufacturers generally rate retention for consumer flash in the range of a year or so for a heavily-worn drive at elevated temperature, and considerably longer for a lightly-used one kept cool. The honest answer is that it varies enormously and there's no reliable way to tell from the outside.
Practical guidance: do not use USB sticks or SD cards as your only long-term copy of anything. They're transfer media and working media, not archival media.
If you do have material on flash storage:
- Copy it to a hard drive now
- Power it on annually if you're keeping it, which allows the controller to refresh cells
- Store it cool, not in a hot attic or a car
The other flash failure mode: sudden death
Beyond gradual charge loss, flash devices fail suddenly and completely far more often than spinning drives do.
The controller — the small chip that manages the memory — is a common failure point. When it dies, the memory chips may be perfectly intact and inaccessible. Recovery is possible but specialized and expensive.
Cheap USB sticks and cards use cheap controllers. This is where the price difference between a bargain card and a reputable one actually goes.
There's usually no warning. Spinning drives often give you clicking noises or slow transfers first. Flash devices frequently work perfectly and then don't work at all.
Reading SD card ratings
The markings are genuinely confusing. Here's what matters.
Capacity class — SD, SDHC, SDXC, SDUC in ascending capacity ranges. Your device must support the class; an older camera may not read a modern high-capacity card.
Speed class — the older markings, a number inside a C symbol, indicating minimum sustained write speed in MB/s. Class 10 means at least 10 MB/s.
UHS speed class — a number inside a U. U1 is 10 MB/s minimum, U3 is 30 MB/s minimum.
Video speed class — V6, V10, V30, V60, V90, where the number is minimum sustained write speed in MB/s. This is the rating that matters for video recording.
Bus interface — UHS-I, UHS-II, UHS-III, indicated by the number of pin rows. UHS-II cards have a second row of contacts and are much faster, in devices that support them. In devices that don't, they work at UHS-I speeds.
The number printed largest on the card is usually maximum read speed under ideal conditions, which is the least useful figure. Sustained write speed is what determines whether your camera drops frames.
What to buy: for photography, U3 or V30 is comfortable. For 4K video, V30 minimum, V60 for higher bitrates. For general file transfer, anything.
Counterfeit cards, which are widespread
Fake memory cards are a large and persistent problem. They're programmed to report a large capacity while containing much less. Everything appears to work until you exceed the real capacity, at which point new data silently overwrites old data or vanishes.
The failure mode is particularly cruel: you fill a card with photos over a holiday, and the early ones are gone.
Protecting yourself:
- Buy from the manufacturer or a retailer you trust. Marketplace listings from unknown sellers are where most fakes come from.
- Be suspicious of the price. A high-capacity, high-speed card at a fraction of the usual price is not a bargain.
- Test new cards before relying on them. Free tools exist that write and verify data across the full capacity. Run one on any new card before a trip.
- Check the packaging and printing quality. Fakes are often close but not exact.
Testing a card takes an hour of unattended time and it's the only way to be sure.
How long things actually last
Rough guidance, with wide variation:
USB sticks — the least reliable category. Cheap controllers, cheap memory, and often physically fragile. Treat as disposable transfer media. Assume anything on one is temporary.
SD and microSD cards — better than USB sticks from reputable manufacturers, and still flash. Cards used in cameras get heavy write cycling and wear out. Replace working cards every few years, and immediately at the first sign of an error.
External SSDs — the best flash option. Better controllers, wear levelling, and more spare capacity. Good for active use and travel. Still not ideal for long unpowered storage.
Internal SSDs — rated in terabytes written, and modern drives have endurance far exceeding what most people will ever write. For normal use, an SSD will be replaced for being small or slow long before it wears out.
Spinning hard drives — better for long unpowered storage than flash, and still not permanent. Replace every three to five years for active use.
What to use for genuine long-term storage
Two spinning hard drives, in two locations, checked annually. Unglamorous and it's what most people should do. Plug each in once a year, confirm it reads, copy anything new.
Cloud storage or a cloud backup service as one of the copies, which removes the physical failure question and adds a dependency on an account.
Archival optical discs, for a very specific use case. Purpose-made archival discs are rated for decades and are genuinely stable, but they're expensive per gigabyte, require a compatible drive, and drives are becoming scarce. Worth considering only for a small, highly valuable, write-once archive.
Not: ordinary writable CDs and DVDs, which degrade unpredictably and often within years. Not USB sticks. Not SD cards. Not a single drive of any kind.
The rule that covers everything: no single medium is permanent. Longevity comes from having multiple copies and checking them, not from choosing the right material.
Practical habits
Card and drive labelling. Write what's on it and when. An unlabelled drawer of USB sticks is a drawer nobody will ever go through.
Eject properly. Pulling flash storage mid-write corrupts it, and flash is less forgiving than other media because there's no journaling to recover from.
Format in the device that'll use it. Cameras in particular prefer to format their own cards.
Retire cards on the first error. A card that produces one corrupt file will produce more. They're cheap; the photos aren't.
Don't reuse a card immediately after offloading. Keep the images on it until they're in two other places. A card is a free extra copy until you need to reuse it.
Rotate camera cards. Several smaller cards beat one large one — if one fails, you lose part of a trip rather than all of it.
The summary
Flash is for moving data and for active use. It is not for keeping data.
If something exists only on a USB stick or an SD card, it's not stored, it's parked — and the meter is running.
Next in this series: How to Organize Files on an iPad or Tablet