My business Dropbox was working perfectly until today when I had to disconnect my hard drive (I have my Dropbox on an external drive, not ideal I know but it's worked fine up to this point). Upon reconnecting the drive, suddenly all the folders I had selected as online only on Smart Sync are being downloaded locally. I am a video editor and we use Dropbox for file storage and sharing, so this is many TB of data that I don't need and don't have the room for on my computer, hence the need for Smart Sync. Furthermore the files that I need to receive for my project today are at the bottom of the pile as it downloads all of these tens of thousands of other files I don't need! Even using Selective Sync to attempt to hide the files from my computer doesn't seem to work, it still is suddenly trying to sync 3 TB of data that I had set as online only. How do I fix this?? It only lets me select smart sync for some folders but not all, and no matter what I do the number of files syncing doesn't seem to change.
... I had to disconnect my hard drive (I have my Dropbox on an external drive, not ideal I know but it's worked fine up to this point).
It's not a matter of it not being ideal. It's a risk, plain and simple. If the drive is disconnected while Dropbox is running, it can see that as a mass deletion, which it will then sync to your account online, deleting all the files that it can no longer find because the drive is missing.
At this point, your best bet will likely be to uninstall Dropbox, rename your existing Dropbox folder to Dropbox.old, and then reinstall Dropbox and start with all of your files marked as Online-only. Once the install is done and Dropbox has synced your complete file list, mark the files and folders you need right away as Local and let those sync. After you have the files that you need right away, go back and set the other files and folders that you want as Local and let them all sync.
And if you insist on continuing to run Dropbox on an external drive, I would highly recommend having a backup of your data, including all of the files that you have marked as Online-only.