To successfully extract the contents, you must have of the archive. If you only have the .001 file and parts .002 or .003 exist, the extraction will fail with an "Unexpected end of data" error. How to Extract .7z.001 files - Microsoft Q&A

A file ending in is the first part of a split compressed archive created with 7-Zip . Large files are often broken into smaller numbered pieces (like .001, .002, .003) to make them easier to upload, email, or store on removable media. How to Open a .7z.001 File