I knew I had read something to this effect in the past since one of my raspi‘s had this thing where you couldn’t write to a mountpoint unless the drive/USB disk was actually mounted.

Since I spent some time in finding it again, here I add it to my blog as personal reference.

From this post:

Always set the attributes of mountpoint directories to immutable using chattr.

This is accomplished with chattr +i /mountpoint (with the mount unmounted).

This would error-out on new write activity and also protects the mount point in other situations.