When working with low level SD drivers, there are few things you need to do in order to get anywhere near the actual write speeds advertised on cards.

MULTIPLE_BLOCK_WRITE (CMD25) is probably the most important things you can do. This needs to be combined with SET_BLOCKLEN (CMD23) in order to tell the internal logic in the SD card about our intention to write more than 1 block.

Here is an example of writing a 512kb file to an SD card. (1 bit mode, 12MHz clock, exFAT FS)

About 2.5s, this results in a write speed ~204kBytes/s. When running in 1bit at 12MHz our bus speed is ~1500kBytes/s. Even with overhead of filesystems we should be able to do better.

Lets switch to using CMD23 and CM25.

For clarity the scale remains to same. This time it took 0.45s, which results in ~1100kByte/s write speed. MUCH better!

You will notice that the CMD line is active during the transaction, this indicates a start/stop of dataflow. Why? This is due to the structure of the file system. By default FatFs will only write continuous streams until you hit a cluster boundary. In this case this card was formatted with a 32kb clusters. this results in 16 separate transactions.

Every one of these transactions incurs a 1ms write time when the card is busy, and can't be used. FatFs includes a command that lets you pre-allocate continuous space for a file. f_expand. If we use f_expand then we can perform all our filesystem tasks in one go. then have a free-run to write the file out.

Total time 0.42 = 1200kBytes/s

We are still operating in 1bit mode, 12MHz. All we have done is alter the firmware. I'm using high quality Samsung cards that have a stated max write speed of 60MB/s. You can see that we don't incur any delays while writing all the main data for the file.

In order to reach a 60MB/s write speed you require hardware that can switch the signalling to the SD card into UHS mode. This uses 1.8V signaling, instead of the standard 3.3V.

As my hardware does not contain this additional hardware, so I'm limited to using HS mode. 50MHz 4bit which should enable near to 25MByte/s.