If you received the following error message, it's possible Dropbox can't sync files to your computer because your computer's hard drive is full.
For Dropbox to sync completely, you need at least the same amount of hard drive space free as the amount of storage in your Dropbox account.
Dropbox will sync as many files as possible to your Dropbox folder on your hard drive until it reaches capacity. Once it is full, you will have to make room on your hard drive to sync more files. Otherwise, even if your hard drive is full, your files are always available for access from the Dropbox website.
If you find yourself with a hard drive smaller than your Dropbox, which is more likely with netbooks or other portable computers, you can always choose which Dropbox files to sync to your computer via Selective Sync. Your Selective Sync settings can be found via your Dropbox desktop application settings.