Backblaze does not support Volume Shadow Copy - When a file is locked or "in use" Backblaze will not be able to read it and create the temporary copy of the file needed for the upload process.
If Backblaze is attempting to upload a file in use, the file will be skipped until the next attempt to upload (approximately one hour).
You can check what files are being skipped by opening your Backblaze Preferences (Mac),or Backblaze Control panel (Win), and selecting 'Settings' and choosing the Reports (Mac) or Issues (PC) tab as noted below. This will provide a list of the files that Backblaze has skipped and will attempt to back up again later. If the file is open in another program it will shows as Temporary File Busy.