Reformat Fusion Drive

Hi. The internal Fusion Drive of my 2019 was way too full and consequently very slow. I acquired a new external Samsung SSD and have made that the boot drive. I updated to Sequoia and it boots great. My intention was to then erase and reformat the Fusion Drive to use it as a workspace for photo work. I quickly tried to erase, and reformat. These efforts failed as the drive was “mounted”. I tried to eject that drive but it indicated that one or more programs were accessing it. I had assumed that with booting from the new external Samsung SSD that the Fusion Drive would be free. Is it still connected somehow as it was the original drive? Can I clean it off and use as a workspace and if so, how do I do it? Thx.