Make sure you know what you are saving to DropBox

[Xairbusdriver]

Dropbox and iCloud Issue, Tidbits discussion
Basically, a file you have stored in iCloud may/can write a file to your local drive that is nothing more than a pointer/link to that file in iCloud. It will be very small (the minimum size is now ~4k, I think, whatever the actual Sector size in the file system). Obviously, this saves lots of space on your hard drive by storing the actual multi-GB file in iCloud.

However, the System may/can write a file on your drive with the same name as the original and iCloud copy. The difference is that it will have a suffix of ".iCloud". If you don't allow the System to display file 'types' (suffixes) you will probably not notice the anything. If you now drag that ".iCloud" file to your DropBox folder, the only thing stored will be the "tiny" ".iCloud" file rather than the multi-GB file you thought you were sending.

The worst result is that iCloud may see that you have moved the ".iCloud" file from where it should be and 'think' you have deleted the file. No problem... except that iCloud will now delete the copy you had stored at iCloud! And, since all you moved to DB is the little 'pointer' file, the only copy you might have is the original or a copy on some local external drive!

The best solution (IMHO) is to not try to store the same file in both iCloud and DB (or any other 'cloud server'. Secondarily, disable the suffix-hiding method in Finder: Prefs->Advanced->"Show all filename extensions".
Ahile you have Finder prefs open, I'd suggest also checking the "Show warning before changing an extension" and "Show warning before removing from iCloud Drive".

[kimmer]

How does a user stop iCloud from saving files? Pretty sure I'm set so that all files are saved on my HD, but with the way things are changing without letting folks know, now I'm not sure. Is there a pref I should check? It's been way too long since I fixed all this for me to remember. 

[Xairbusdriver]

In macOS 10.12 Sierra and macOS 10.13 High Sierra try checking the System Prefs->iCloud. There should be a list of apps that you can allow to use with iCloud Drive or even disconnect from it entirely. Along the bottom of the window is a bar graph showing how much free space is available and some of the apps that have use the most space.

I like to review all System Prefs after every new install. Same for major updates to important apps. Many times the choices in preference lists change or get new features. You probably won't use them if you don't know about them.