diskutil list

reformatting the disk did not also change the partition map scheme. Well, the setting for this is hidden in an Options dialog in Disk Utility’s Partition tab, and I never went there when I reformatted the drive. I just clicked on the Erase tab, selected Mac OS Extended (Journaled) as the volume format, entered a volume name, and clicked Erase. This apparently does not change the disks partition map scheme.