iCloud & iCloud Photos · Storage Studio and iCloud
Does Storage Studio need Full Disk Access to work with iCloud Drive?
Short answer: Yes. macOS protects Desktop, Documents and the Library folder holding iCloud Drive, so Storage Studio needs Full Disk Access to measure and free them. It sees folders on your Mac, never your iCloud account.
Yes, for its full set of features. macOS protects folders such as Desktop, Documents and the Library folder that holds iCloud Drive, and Storage Studio needs Full Disk Access to measure them, look for duplicates in them, and free their downloaded copies.
Grant it in System Settings → Privacy & Security → Full Disk Access, then switch on Storage Studio. The app's setup walks you through it.
Full Disk Access lets Storage Studio see these folders on your Mac; it doesn't give it access to your iCloud account, and it never asks for your Apple Account password. See privacy and safety.