Copying Apple's system is now an Apple-proprietary endeavor; we can only offer "best effort" support for making an external bootable device on macOS Big Sur (and later OSes). We present this functionality in support of making ad hoc bootable copies of the system that you will use immediately (e.g. when migrating to a different disk, or for testing purposes), but we do not support nor recommend making bootable copies of the system as part of a backup strategy.| bombich.com
While some developers seem surprised by a change in macOS 15.2, we've known for several years that making bootable backups would eventually become impossible. We shifted CCC's strategy away from relying on External Boot so our users wouldn't be affected by this inevitable result.| bombich.com