First install udevil , so that autofs gets notified when new usb devices attach.
A few notes:
exfat fs is not included in core or ntfs-3g, so install exfat-fuse package on archlinux
autofs append "-s" parameter to ntfs-3g and mount.exfat, that broken mounting process, the workaround would be replace these two binaries with bash script, remove "-s" then invoke the real binaries. (credit)