I'm trying to understand what is meant by the wording of this Disk Utility cautionary message that appears when attempting to add a new partition to a disk.
Partitioning this device will delete the data stored on some of the partitions, and cannot be undone. This partition will be added: "Secondary" This partition will be erased: "Secondary" This partition will be resized: "Primary"
First point of confusion: It's going to be added and then erased?
I added a new partition and everything appears to have gone smoothly. An answer to another similar question helped me to understand and decide to move forward with partitioning.
This guide, How to Use macOS Sierra Disk Utility to Partition, Erase Drives was also helpful.