Yes, there is the rumor that the flash memory used for micro-SD cards is kind of scrap. And it’s probably true. Due to this and because speed matters, I buy more expensive SD cards such as the SanDisk ‘Extreme’ line. But even those cards start to slow down and show erratic behavior after a few years of usage.
Here’s a case in point: For many years, I’ve been running a Raspberry Pi at home to act as a gateway. A SanDisk ‘Extreme’ 32 GB served as the boot device. When upgrading to a newer Debian version, I first made a copy of the SD card with the ‘dd’ utility. Already there, I noticed the slow and erratic read behavior. Later, I restored the backup to this SD card and again noticed an erratic and slow write behavior. I then compared the write speed with another SD card, same make and model, about the same age, but almost not used at all over the years, and the difference was significant. Instead of ‘slow and erratic‘, I got ‘fast and stable‘ write rates of about 60 MB/s. That’s a factor of 5 faster than the ‘well used’ SD card. The screenshot above shows the difference. The left two thirds of the graph shows the write speed on the well used card, the right third of the graph shows the write speed to the other card, same model, but much less used.
So long story short: Even more expensive Micro SD-cards don’t live forever 🙂