There is an issue in Apple’s OS X Yosemite that causes problems - TopicsExpress

There is an issue in Apple’s OS X Yosemite that causes problems with Dropbox. You can resolve this issue by installing the latest Software Update for OS X Yosemite (10.10.1). Please dont install any software update without having a tested and working backup system in place. OS X Yosemite may occasionally cause some programs to crash when you open, save-as, or first save a file. These crashes are rare but happen when an application, such as Dropbox, uses Yosemite’s official Finder integration — and if that program crashes because of this interaction, unsaved changes may be lost. To fix this issue, Apple has released OS X Update 10.10.1. This update is available for free in the Mac App Store.