I have 2 directories which both have stuff in them:

• /home/user/folderApple

• /mnt/drive/folderBanana

I want to mount folderBanana onto folderApple like this:

sudo mount --bind "/mnt/drive/folderBanana" "/home/user/folderApple"

But I still want to be able to access the contents of folderApple while this is activated. From what I am reading, binding the original directory to a new location should make it available, like this:

mkdir "/home/user/folderApple-original"
sudo mount --bind  "/home/user/folderApple" "/home/user/folderApple-original"

But this just binds /mnt/drive/folderBanana to /home/user/folderApple-original as well. I tried reversing the order and result is the same.

How do I tell mount to look for the underlying directory?

I am happy to use symlinks or something else if it'll reliably get the job done, I am not wedded to this mechanism.

(The purpose of all this is that when an external drive is connected, I can have the storage conveniently available, but when it is not connected, the system will fallback to internal storage. But then I will want to move files between the fallback and external locations when both are available. So I need to see both locations at once.)