If you’re like many of my clients who use Dropbox intensively, you have a desktop Mac with a large drive and a MacBook with much less drive space. How do you prevent your large Dropbox account from overwhelming the laptop Mac’s available storage? The answer is Dropbox’s Selective Sync feature. On the MacBook, click the Dropbox icon in the menu bar, click your avatar in the upper-right corner, and choose Preferences. In the Preferences window, click Sync and then click the Choose Folders to Sync button. Deselect the folders you want to prevent from syncing to the MacBook and click Update. If you need to access any files in those folders from the MacBook, go to dropbox.com in your Web browser instead, or adjust your Sync preferences to bring in the needed folder.
If you are on a higher tier plan from Dropbox you can instead enable “Smart Sync” which creates your entire Dropbox structure on your computer, but if you begin to run out of local storage files that have not been accessed in a while will get replaced with dummy files. Opening the file will trigger Dropbox to download the full file to your computer and then open it. In other words, it will use your Dropbox account as augmented cloud storage so that it’s like you have more space on your computer.
If you use Dropbox on iPad or iPhone you don’t need to worry about this. Dropbox for iPad and iPhone will only store files you explicitly ask it to.