Posted on January 29, 2016 at 10:17You will have a much better model of how an SD card works if you imagine that you are interfacing to a computer (and not memory). SD card's are capable of high performance, but not always when you want it. You need to lower your expectations, and would be better off if you can have half a second (or even more) of buffering when writing to the SD card.
1) Writing large amounts of sequential data is generally quite efficient. You should be able to get sustained write rates of almost megabytes per second.
2) Writing smaller amounts of sequential data (and expecting the sort of response that you want) is nowhere near as efficient - as you've already found!
3) Reversing direction and writing bytes earlier in a memory sector after having written bytes later in the sector is a disaster (100's of milliseconds of dead time as the SD ''computer'' deletes and rewrites the sector).
This is true any time you access SD memory - it is not unique to the STM32, but is a characteristic of how SD memory works.
If at all possible - stick with writing large blocks of sequential data!
