External drive backup not being offered

[GreatLand1959]

According the help files:

To back up an external hard drive to your Dropbox account:

Open the Dropbox desktop app on your computer.
Connect your external hard drive to your computer. A Dropbox window will appear, prompting a back up of your external hard drive.

Except I have an external drive which Dropbox doesn't seem to want to let me add to backup. I've repeatedly detached, then reattached the drive with no success. How do I force Dropbox to recognize the drive and add it to the backup function?

[Walter]

Hey @GreatLand1959, thanks for using Dropbox and welcome to our Community!

Could you please clarify the OS of your device and the file-system and size of the external drive you're trying to backup?

This will help me assist further. 

[GreatLand1959 · Accepted solution]

High Sierra, 5tb external. I was finally able outsmart Dropbox and force it by copying a hidden file from another external drive that Dropbox saw with no trouble. But it shouldn’t be this difficult. It should be a matter of pressing a button to add a drive, the select it from a dialogue box.

[kcristony]

Can you explain how did you do it? i would like to make this work again