A backup is useful only if the files can be recovered and opened. Thumbnail projects often depend on linked assets that are easy to omit from a simple copy of the main project file.
Collect the full project
Include the editable document, linked images, final exports, and source notes. Store font information and licences where appropriate. Keep the folder structure stable so links remain understandable after restoration.
Maintain a separate copy
Use a backup location independent of the working folder, following the storage approach appropriate to your project. Keep important approved versions rather than replacing the only backup with every experimental edit.
Test a restoration
Copy one project into a temporary location and open it. Check for missing assets, changed text layout, and unreadable files. Compare its export with the archived approved image. Record any missing dependency and correct the backup process. A small practical restore test reveals problems that a folder-size check alone cannot catch.