Have root & external sdcard mounted rw, but mv produced "Permission Denied"
@IrfanLatif offered a comment that included an excellent link to all the things root can't do.  It turns out, reading a FUSE filesystem owned by another user, is one of them.  Android, will mount a device at multiple places, each with different permissions.  Going to /storage/[UUID], you are visiting a FUSE filesystem.  On the other hand, going to /mnt/media_rw/[UUID], you are visiting the filesystem directly.  There may, in addition, be SELinux permission issues, as described in that link, but in my case those did not appear.
$ mount | grep 1dc31997
/dev/block/vold/public:179_97 on /mnt/media_rw/1dc31997-9ebf-4573-9c75-3e57793b4071 type f2fs (rw,dirsync,context=u:object_r:sdcard_posix:s0,nosuid,nodev,noatime,background_gc=on,user_xattr,active_logs=6)
/dev/fuse on /mnt/runtime/default/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/read/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/write/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)