It is a little bit arguable if multithreading offers any speed benefits when dealing with writing or reading. Typical HDD of a PC can only do 1 task at the time. It has an internal queue of operations, and if your app would send sequential writing tasks split in many, it would be slower than 1 bigger task, propably not very noticeably slower though. But if it's SSD, it would be able to do them simultaneously. I don't know how these translate to Rasperry PI. I can only guess that MicroSD is different tech than SSD, and i can't really find any details on it.
But in all cases there should be a data buffer on hardware which is the one apps communicate with. You can't directly access the hardware's reading or writing in that low level.