March 11, 2018. An update and clarification: You can use the .nosync extension to prevent a folder or file from being synced to iCloud drive. For example, you can rename Osirix Data folder to OsiriX Data.nosync. For a file, you can put the nosync directive before the file's extension. For example. for a PDF file titled test.pdf, use test.nosync.pdf This is a good practice to prevent issues with apps that look for the usual file extensions.