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?