Can't open file on an SD card with .open() from SDFat.

Other Hardware Storage
2

It may be due to the dimension of

char TxFileName[11];

This is really only room for ten characters since there must be a zero byte to terminate a string.

This string requires 12 bytes when you include the zero byte.

char TxFile[] = "GR_TX00.csv";

This will overwrite an extra byte somewhere.

strncpy(TxFileName,TxFile,sizeof(TxFile));

The same is true for RX files. This may not be the open problem but it will bite at some point.