Hi:
The USB doesn't have the implementation to have a R/W directly without a file system and the conclusion is that it is better for you use SD and explore using the IO driver and using the examples provided in the document MQXIOUG.pdf.
We already have an ESDHC driver.This driver defines common interface for communication with various types of cards including SD, SDHC, SDIO etc. The driver is currently used as an alternative to SPI low level communication for SDCARD wrapper under the MFS stack. You can find the example of using the driver in section 16.11 of MQXIOUG.pdf document.
Regard
Daniel