That's one of the real headaches of Cryptolocker, apparently. If you have automatic backups set up, the encrypted files will most likely be backed up there and overwrite the clean ones. To really be safe, you need either secure versioned backups - default Windows and MacOS backups may not work, since the drive is connected and fully accessible to the computer - or to do backups manually to media not normally connected to the computer, and notice that something has gone wrong sometime between when the infection starts and when you connect the drive. Did I mention yuck?