from ff.h:85
typedef struct
{
BYTE fs_type;
BYTE drv;
BYTE n_fats;
BYTE wflag;
BYTE fsi_flag;
WORD id;
WORD n_rootdir;
WORD csize;
#if _MAX_SS != _MIN_SS
WORD ssize;
#endif
#if _USE_LFN != 0
WCHAR* lfnbuf;
#endif
#if _FS_EXFAT
BYTE* dirbuf;
#endif
#if _FS_REENTRANT
_SYNC_t sobj;
#endif
#if !_FS_READONLY
DWORD last_clst;
DWORD free_clst;
#endif
#if _FS_RPATH != 0
DWORD cdir;
#if _FS_EXFAT
DWORD cdc_scl;
DWORD cdc_size;
DWORD cdc_ofs;
#endif
#endif
DWORD n_fatent;
DWORD fsize;
DWORD volbase;
DWORD fatbase;
DWORD dirbase;
DWORD database;
DWORD winsect;
BYTE win[_MAX_SS];
} FATFS;