def METHOD_NAME(check_dir):
    """Check if "check_dir" is on FAT. FAT considered harmful (for big files)
    Works for Linux, Windows, MacOS
    NB: On Windows, full path with drive letter is needed!
    """
    if not (os.path.isdir(check_dir) or os.path.isfile(check_dir)):
        # Not a dir, not a file ... so not FAT:
        return False
    FAT = False  # default: not FAT
    # We're dealing with OS calls, so put everything in a try/except, just in case:
    try:
        if "linux" in sys.platform:
            # On Linux:
            # df -T /home/sander/weg
            """
            Example output of a 500GB external USB drive formatted with FAT:
            $ df -T /media/sander/INTENSO
            Filesystem     Type 1K-blocks      Used Available Use% Mounted on
            /dev/sda1      vfat 488263616 163545248 324718368  34% /media/sander/INTENSO
            """
            dfcmd = "df -T " + check_dir + " 2>&1"
            for thisline in getcmdoutput(dfcmd):
                if thisline.find("/") == 0:
                    # Starts with /, so a real, local device
                    fstype = thisline.split()[1]
                    if debug:
                        print(("File system type:", fstype))
                    if fstype.lower().find("fat") >= 0 and fstype.lower().find("exfat") < 0:
                        # FAT, but not EXFAT
                        FAT = True
                        if debug:
                            print("FAT found")
                        break
        elif "win32" in sys.platform:
            import win32api
            if "?" in check_dir:
                #  Remove \\?\ or \\?\UNC\ prefix from Windows path
                check_dir = check_dir.replace("\\\\?\\UNC\\", "\\\\", 1).replace("\\\\?\\", "", 1)
            try:
                result = win32api.GetVolumeInformation(os.path.splitdrive(check_dir)[0])
                if debug:
                    print(result)
                if result[4].startswith("FAT"):
                    FAT = True
            except:
                pass
        elif "darwin" in sys.platform:
            # MacOS formerly known as OSX
            """
            MacOS needs a two-step approach:
            # First: directory => device
            server:~ sander$ df /Volumes/CARTUNES/Tuna/
            Filesystem   512-blocks      Used Available Capacity iused ifree %iused  Mounted on
            /dev/disk9s1  120815744 108840000  11975744    91%       0     0  100%   /Volumes/CARTUNES
            # Then: device => filesystem type
            server:~ sander$ mount | grep /dev/disk9s1
            /dev/disk9s1 on /Volumes/CARTUNES (msdos, local, nodev, nosuid, noowners)
            """
            dfcmd = "df " + check_dir
            for thisline in getcmdoutput(dfcmd):
                if thisline.find("/") == 0:
                    if debug:
                        print(thisline)
                    # Starts with /, so a real, local device
                    device = thisline.split()[0]
                    mountcmd = "mount | grep " + device
                    mountoutput = os.popen(mountcmd).readline().strip()
                    if debug:
                        print(mountoutput)
                    if "msdos" in mountoutput.split("(")[1]:
                        FAT = True
                    break
    except:
        pass
    return FAT