Physical Block size is the smallest amount of data that can be written at one time to a disk. Recent versions of Macs use 4096 bytes block size (4k sector size), while the default sector size on earlier Macs is 512 bytes.
When the backup is being recovered to a Mac with a different block size, it is impossible to automatically remap partitions. As the result, the restored system may be unbootable.
The macOS device block size can be determined by executing the following command from a Terminal window prompt: