mount.exfat-fuse mounts drive r/o I'm mounting an external exFat drive on a rooted Android device (Amazon Fire TV/2) using the mount.exfat-fuse program. 

mount.exfat-fuse -o rw,umask=000 /dev/block/sda1 /system/test


directory permissions are 777, but permission denied for anything root tries.

Thanks for any help.
It turned out that SELinux policy on the Amazon Fire TV/2 caused my exFat drive to be read-only when mounted, despite mounting arguments. The SELinux command setenforce can be run with an argument of 0 or 1, setting SELinux to permissive or enforced, respectively. When set to 0, the drive can be used normally.

So, I check for the presence of setenforce in my mount script and set it to 0 if found. The unmounting script does setenforce 1, putting things back as they were.