You can also do the following to prevent unwanted writes when something is not mounted at /mnt/thatdrive:

# make sure it is not mounted, fails if not mounted which is fine
umount /mnt/thatdrive

# make sure the mountpoint exists
mkdir -p /mnt/thatdrive

# make the directory immutable, which disallows writing to it (i.e. creating files inside it)
chattr +i /mnt/thatdrive

# test write to unmounted dir (should fail)
touch /mnt/thatdrive/myfile

# remount the drive (assumes it’s already listed in fstab)
mount /mnt/thatdrive

# test write to mounted dir (should succeed)
touch /mnt/thatdrive/myfile

# cleanup
rm /mnt/thatdrive/myfile

From man 1 chattr:

A file with the 'i' attribute cannot be modified: it cannot be deleted or renamed, no link can be created to this file, most of the file's metadata can not be modified, and the file can not be opened in write mode.
Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.

I do this to prevent exactly the situation you’ve encountered. Hope this helps!