You’re mid-shoot. The camera hesitates, then locks up. The buffer indicator freezes. A warning flashes. You check the card: 4 GB remaining, maybe more. Objectively, there is space. The camera disagrees. This particular frustration is more common than most tutorials acknowledge, and the causes are technical enough that “just format the card” — while sometimes correct — barely touches the surface of what’s actually happening.
What the Camera Sees That You Don’t
A memory card’s reported free space and a camera’s ability to write to it are not the same thing. Every SD, CFexpress, or XQD card maintains a filesystem — FAT32 or exFAT being the most common — and that filesystem has internal structures the camera interacts with separately from the raw storage pool.
FAT32, for instance, imposes a hard ceiling of roughly 4 GB on any single file. A camera recording video in a long-take or writing a large-buffer burst to card will hit that ceiling and stop, regardless of how many gigabytes remain. The card isn’t full; the file limit has been reached. Cameras writing in RAW + JPEG simultaneously can also exhaust the FAT32 single-file limit faster than shooters expect, because both files are being written nearly in parallel.
exFAT removed that single-file ceiling, which is one reason UHS-II and CFexpress cards almost always ship pre-formatted as exFAT. But even exFAT cards can behave erratically if the card was formatted inside a computer rather than in-camera, because the cluster size chosen by macOS or Windows may not match what the camera’s write controller expects. A large cluster size wastes space at the block level, meaning the available bytes visible to the OS don’t translate cleanly into the usable space the camera can address in contiguous writes.
The Buffer Is a Separate Problem From the Card
Many photographers conflate two distinct failure modes: a full card and an exhausted buffer. The buffer is RAM inside the camera — a temporary holding area that stores images while the write processor offloads them to the card. If shooting speed outpaces write speed, the buffer fills, and the camera stops accepting new shots until it drains.
The card’s available space is irrelevant to this calculation. What matters is the sustained write throughput of the card versus the camera’s demanded write rate. A card rated at high sequential read speeds can still have low sustained write performance — a detail buried in spec sheets that is easy to overlook when purchasing. Some consumer-grade cards are engineered to burst fast and write slowly on sustained operations, which matters considerably less for casual shooting and considerably more for burst-mode sports or wildlife work.
The camera stopping with space visible on the card is, in many of these cases, the buffer refusing to drain fast enough rather than the card refusing to receive data. The two look identical from where you’re standing. They require different fixes.
Why the Fragmentation Problem Gets Worse Over Time
A card that works perfectly when new can become increasingly prone to stalling over months of use without ever showing a single corrupt file. The reason is fragmentation — not in the traditional spinning-disk sense, but in how the card’s internal controller maps written blocks.
Flash storage doesn’t overwrite in place. When a file is deleted, the blocks it occupied are marked as available, but they aren’t blanked immediately. The card’s controller manages wear leveling across cells, meaning new writes may get scattered across physically non-contiguous blocks. Eventually, sustained burst writes require the controller to coordinate writes across many disparate block locations simultaneously, which adds latency. The camera’s buffer backs up. The camera pauses.
In-camera formatting — not a computer-side “quick erase,” which often just clears the filesystem index rather than resetting the block map — instructs the card controller to perform a more thorough reset of its allocation tables. This is why the advice to format in-camera rather than on a desktop is technically sound: you’re not just clearing the directory, you’re giving the card’s controller a cleaner state to work from. Whether a given card’s firmware does a full erase during in-camera format varies by manufacturer, but the result is almost always more consistent sustained write behavior than a delete-all on a computer.
For a deeper look at how card format choices interact with camera workflows, the article on migrating from XQD to CFexpress formats covers the write architecture differences between slot types in practical terms.
File Format Choice Has a Direct Impact Here
What the camera is writing matters as much as how fast it’s writing. Shooting RAW produces files that are substantially larger than JPEG equivalents — not as a universal constant, since RAW file size varies by sensor resolution, bit depth, and the camera’s own lossless or lossy RAW compression setting. A camera set to uncompressed 14-bit RAW fills both the buffer and the card faster than one set to compressed RAW, and the difference in image quality between lossless-compressed and uncompressed RAW is often negligible in practice.
If you’re shooting RAW + JPEG simultaneously and the JPEG size setting is at maximum, every shot writes two large files. Some photographers keep that setting out of habit from an older workflow. Switching JPEG size to medium or enabling in-camera lossless RAW compression can meaningfully reduce the write load without sacrificing the RAW file’s editing latitude.
The RAW vs. JPEG tradeoffs covered in our compression workflows piece gets into the specific quantization mechanics that make this a real difference — not just a storage preference.
A Practical Checklist Before Your Next Shoot
When a camera stops writing with space remaining, work through these in order before assuming the card is failing:
- Check the filesystem. If the card is FAT32 and you shoot video or large RAW bursts, reformat in-camera as exFAT (check that your camera supports it first — not all older bodies do).
- Format in-camera, not on the computer. Give the card’s controller a clean allocation state before the session, not just an empty directory.
- Verify sustained write speed, not sequential. Check the card manufacturer’s published sustained write specs for your specific card model, not the headline speed on the package.
- Reduce simultaneous write load. If shooting RAW + JPEG, consider whether you need both at maximum size.
- Match the card to the slot. A UHS-I card in a UHS-II slot writes at UHS-I speeds — fine for casual work, limiting for sustained burst.
- Check buffer depth in your camera’s manual. Some bodies list maximum burst depth per card type. That number is the ceiling, not the card’s space.
- Retire high-cycle cards from burst-heavy duty. Flash cells degrade with write cycles; a card showing no errors but stuttering on sustained writes may be past its useful write performance peak even with plenty of space showing.
None of this requires new gear. Most stalled-card situations resolve at step one or two. But understanding which layer of the problem you’re actually looking at — filesystem, buffer, write throughput, or block fragmentation — keeps you from blaming the card when the real constraint is somewhere else entirely.