Micro SD cards are a great way to expand storage in smartphones. They can also be used in laptops with an SD card adapter, as most have SD card readers. USB flash drives are a good way of adding storage to computers. In the following post, I'll use the term flash drive to refer to both.
But you have to be careful. There are a great many fakes in the retail wild.
I just purchased 2 USB flash drives during Amazon Prime Days. They had 512GB capacity, and in addition to the standard USB A interface, also had Micro USB, USB C and Apple Lightning interfaces as well. On the surface, they appeared to be a great way of transferring data between computer and smartphone.
The first indication of trouble appeared when I inserted one of these in my computer. About 10% of storage is lost due to formatting, so, for a 512GB drive, you should see around 460GB or so available in Windows. This one showed 500GB even (actually, 499.9 but what's 0.1GB between friends). The only times I've seen such even number capacity is in fake drives, where the manufacturer is too lazy to calculate a more reasonable number.
There are a number of software apps available that will test out flash drives/SD cards for capacity. The manufacturers of most fakes usually zap the controller on the drive to show far more capacity than what is available, i.e., you get an actual 64GB (about 58GB available) drive but you see 512GB in Windows (about 460GB available). These fake drives will still work ... at first, but when you reach the real physical capacity of the drive, data starts overwriting, and you lose all that data (because your file system loses integrity).
The program I use, FakeFlashTest, has a number of operating modes, but I find the simplest is best. In that mode, it writes unique data to various parts of the flash drive, and then reads the data back, verifying it actually read what it wrote. It doesn't take that long, and it's caught every fake drive I've ever owned.
Well, the software didn't even manage to get past the write stage, failing at about 85%. The test is destructive, as it goes directly to the physical sectors of the drive. When I then attempted to reformat the drive, and it failed right away with formatting errors.
The same test was performed on the other drive I purchased, and it failed at the same spots.
I now only buy these kinds of things from Amazon, as they have a no questions asked return policy. Back the fakes go.
Postscript: What's frightening is that I purchased these drives because every single review (23 at the time) was 5 stars. Obviously, all these purchasers were just plunking the drives as received in their computer and using them. There was not one mention of checking them out beforehand. Will they ever get a surprise at some point!