No dialog box when connecting external HD?

I'm having the same problem.

I have a Western Digital Passport drive, which I completely deleted and reformatted to HFS+, making sure it was non-encrypted.  At first the dialog box popped up, so all seemed fine.

Then after re-copying the data to the external drive, Dropbox won't recognize the external drive to actually back it up.

I have re-started my machine, restarted the dropbox app & sync, I have ensured the "Show setup notifications when new external drives are plugged in." box is checked.

How can I get, or force the Dropbox app to recognize this drive so that I can get it to start backing up some of the folders on it?
There is nothing in any menu in the preferences etc. to do this, which makes me concerned that even if I am able to sync it at some point the app won't recognize the drive and all the time invested in syncing a bunch of data will be lost if I can't have a manual and sure-fire way to make sure the app is knowing to be getting data from a certain external hard drive.

Can anyone please assist with a way to ensure an external drive dialog box comes up, as I've ensured everything is good to go on my end in terms of formatting etc.