Is "The disk copy of 'filename.psd' was changed since..." dialog a sign of harddrive issues?
Most likely it's Dropbox. It could be that it synced to another machine, which added some metadata to it automatically, then synced back to your machine, so it looks to Photoshop as if it's changed, when only the file's metadata has. Unless this happens consistently with files outside of synced folders, it's probably nothing to worry about