Backblaze creates temporary copies of large data files (over 100MB) during the backup process. You can now move this file to an external drive, or another location, if space is limited on the default boot drive.
Note: Prior to version 6.1 of Backblaze, a "large file" was considered one over 30MB in size.
You can change the location by opening the Backblaze Control Panel or Backblaze System Preferences (click on the Backblaze icon) and pressing the settings button. From there, you can make the change to the Temporary Data Drive.