I have an indesign book and chapter files. I have been backing up (I thought) by copying the folder that contains all of these (chapters and book). When I moved the backup folders of old versions of the book, then tryied to open my current version, I have only text boxes. No placed images, no links to missing images.
The .indb file is a collection of pointers to the individual documents, and any copies you make continue to point to the original locations, and those files continue to point to the original links. If you want to make a backup, use the Package command to make copies of everything and change the link and pointer locations.