Backblaze will not backup any directory at the root level named 'Windows.old'. "Windows.old" has a special connotation from Microsoft, in that it's the name of the folder that's used after an upgrade to Windows. See more information: http://www.thewindowsclub.com/delete-remove-windows-old-folder
To ensure that we do not unnecessarily use your bandwidth, Backblaze excludes temporary and transient data, such as caches, other backup software, the operating system itself, and program files. It does them by directory name at the top level of each drive, so if you have any folders named Windows.old all of the data in this directory will not be backed up. For this reason, we recommend that you do store any important data in this directory, as there is no way to remove this exclusion. If you would like to learn more about our exclusions you can do so here.