It may come as a surprise, perhaps, but there's a lot we have to talk about here.
First up, we have physical mechanism errors. Compact Flash cards are notorious for this, as the card has to slip into a connector that has exposed pins sticking out. The most common thing to happen is that a pin in the connector gets bent or broken, usually because the user at one point or another used too much force, and then all heck breaks lose the next time the camera tries to write to the card.
SD cards have a write protect mechanism that's easy to break, plus I've had SD card shells literally lose their glue and the card come apart in my camera. Putting the card "back together" doesn't always align things properly, and then that card is toast.
XQD cards probably have the most robust, mechanically reliable design of any cards on the market. I've not seen or heard of any physical mechanism errors with XQD.
Second, we have to talk about logic. Cards are treated much like a little hard drive. They have file allocation tables, directory tables, and data stored in sectors scattered throughout the media (which are tracked by that directory and file allocation table). Over time, we've had a number of varieties of FAT and exFAT, as well as other logical structures that are used with drives (and thus cards).
Most "old" card media is using FAT. Most recent card media is using exFAT. Most cameras can recognize the two and adjust, but something is lost in the process of using old media with new process (e.g. FAT media on an exFAT capable device). The whole common instruction about "format your card in the camera only" is part of this.
One thing I don't like to do is share media between cameras, particularly of different makes. Many of the card issues I have encountered have occurred when I've done this, which implies that camera makers may be interpreting or dealing with the logical constructs on the media differently. My advice? Get new cards with your new camera and use them for only that camera. People who follow that advice seem to have fewer issues than those that don't.
Another thing that comes up is how cameras handle deletion and fragmentation on the media. Both JPEG and raw files are not specific in size, but will be a size determined by the amount of detail in the scene. Record a photo of a plain blue sky and the file size will be smaller. Record a photo of the grains of sand on a beach and the file size will be larger.
What that means is that when you delete a photo from the middle of a shooting session during review, you're opening a gap in the media. But that gap may not be large enough for the next photo you shoot, meaning that the data for that next photo will be fragmented (scattered in different places across the media). (It's not quite that simple, as there are load balancing mechanisms in modern media that scatter data, too.)
Camera's have to be ready for "disaster scenarios." You filled the card with images. Then you went back and removed one and tried to shoot another. Did it fit? ;~)
Early in the digital camera era I'd say there were a lot of cameras that didn't handle deletion and fragmentation very well. Indeed, my common recommendation for a very long time was that you never deleted in camera, as the risk of the underlying file structures not surviving correctly during multiple deletes and re-use was high.
These days, I think all the camera companies have matured their camera OS to the point where structure integrity stays high even with multiple deletes and re-use. Still, fragmentation can cause write speed issues.
Which brings me to the most commonly seen error: lines in the photo or an entire area of the photo across the long axis that is a single color, such as purple. This almost always indicates that something didn't go correctly during the writing of data to the card. It can be caused by problems with the write mechanism in the camera, and it can be caused by the media not working at the speed assumed by the camera. The right thing to do in this case is try to isolate: is it only one card that does this, or do all cards do this? In the former case, retire the card (more on that later), in the latter, you probably need to send the camera in to be checked.
When you put a card you shot with into a card reader and get error messages or file not found errors, the first thing you have to do is make sure that you've got the right reader with the correct device drivers installed. It also helps to have the current Digital Camera Compatibility file (macOS) or CODEC (Windows) installed if you're shooting raw files.
But let's say that you know that the reader is fine and the right drivers/support files are present, but you still get an error trying to read files from the card. That's the point where you need to get file recovery software looking at that media. There's a very good chance that the file allocation table or directory has been corrupted on that card and needs to be rebuilt. It's also possible that there's a sector error that wasn't marked as bad by the OS.
Finally, we have the thing most camera users simply don't understand: cards are not forever.
The NAND cells in all memory cards have a limited number of writes they can sustain before becoming unreliable or unusable. Let me scare you a moment by stating that number is usually something around 1000 writes. Wait, what? Only a 1000?
Don't panic. That doesn't necessarily mean that you can only shoot with that card a 1000 times. I suppose if you filled it every time you shot, then yes, that might be the actual implication. But most of us don't do that. Still, the thing that you absolutely need to know is that a card will eventually start having failures of individual memory cells. That problem will get worse the more you use the card.
The good news is that the device/card makers know this, and have put some load balancing and sometimes over provisioning properties into their systems so that the eventual problem of cell unreliability happens later, not sooner.
Which brings me to my advice:
- Buy new cards. Buy several cards that match the capability of any new camera you buy the day you buy the camera (e.g. UHS-II cards for a new Fujifilm X-T3, XQD cards for a Nikon D5). Don't opt for slow cards: make the card match the camera's ability in terms of maximum write speed. Use those cards only in that camera.
- Rotate card use. Don't keep using and formatting the same card. Rotate your card usage between the set of cards you bought in the above bullet. Number them. Use them in sequence.
- Retire cards regularly. If you're buying new cameras every few years, that will happen naturally if you follow my first bullet. If you keep using the same camera for as long as it works, well, at some point you're likely to have a card failure if you keep using the same card.
Unfortunately, everyone I know tends to do the opposite of what I just wrote. They don't buy new cards with a new camera. They try to keep using their old cards. In fact, they'll complain if the new camera has a different type of card slot than their last camera. Then they keep using the same card over and over again in their camera, typically their largest capacity one (eggs all in one basket, folks). And they never retire those cards, at least not until the failure comes that has them losing images.
This is all a bit like buying a car without tires and putting your old car's tires on the new car, then driving the car until the tires burst and you crash.
