Can't write to ntfs file system

I have an external hard drive that is in ntfs file format. Vim will
neither create a file, nor write to existing files, on this file
system. It returns E212, saying I do not have permission.

The drive mounts automatically from my fstab entry when I start the system

/dev/sdb1 /media/500gb ntfs-3g rw,user,auto 0 0

and mount shows it as

/dev/sdb1 on /media/500gb type fuseblk
(rw,noexec,nosuid,nodev,allow_other,blksize=4096)

so its permissions and ownerships are universally, e.g.

/media/500gb/Films/Mouchette (1967) (French with English Subtitles) $ls -l
total 4
drwxrwxrwx 1 root root 4096 Jan 1 08:50 VIDEO_TS/
-rwxrwxrwx 1 root root 0 Jan 1 10:40 mplog*

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