I was looking up the SD library functions here https://www.arduino.cc/en/Reference/SDopenand the documentation for the open() function is confusing.
It says
Syntax
SD.open(filepath)
SD.open(filepath, mode)
Parameters
filename: the name the file to open, which can include directories (delimited by forward slashes, /) - char *
Either filepath or filename should be used. I'd recommend filepath with a fuller explanation and an example such as:
filepath: the name the file to open, which can include directories (delimited by forward slashes, /) for example "dir1/dir2/MyFile.txt"
Peter.