You cannot easily modify a Solid archive. Because the files are interwoven into a single data stream, inserting or deleting a file breaks the chain. To update a solid archive, WinRAR essentially has to recompress the entire affected region. This is why Solid archives are best for "Write Once, Read Many" (WORM) scenarios, like software distribution or long-term backups.
In a standard archive (like a standard .zip or a default .rar without the solid flag), the compression algorithm treats every file as an isolated island. winrar full
