so lets start simple,
have you tried a different sd card? or tried a different sector? (not all SD cards are equal)
have you made sure the card lock switch is in the correct position? (im sure you have but just want to make sure.)
are you able or do you know how to use "dd" to see if you actually have written to the sd card? (dd is a command to either write or read raw data from a disk or image.) we want to verify that you are actually writing to the sd card
The code looks good, but i dont use RTOS, and dont have a way to test at this time, im sure i could but I dont have time to get it wall setup.
I can only give you limited support, and hopefully point you in the right direction.........
Your HW does have SPI, so you should be able to write to SD cards with out issue.
Try looking here
the only thing I see is you have to change the code to fit your HW, I do not know if SSI is close to USCI or USI.
also if you add more info on your setup we maybe able to guide you in the right direction as well.