How to debug fopen() returning NULL?

I have an eMMC connected to an STM32H7 via the SDMMC1 interface and am using the File System component provided by Keil.

The eMMC registers as M0:. In the debugger, the File System window shows everything looks good.

finit() and fmount() both return fsOK.

However, after these, when I use fopen("Test.txt", "w") the result is always NULL. 

I do not have access to the source code and there are no events generated when this call fails.

How can I debug this?