External drives · Drive formats
How do I add a second APFS volume to an external drive?
Short answer: In Disk Utility, select a volume on the APFS drive and click + (Add Volume). The new volume appears as its own drive without erasing anything, and all volumes share the drive's free space unless you set a reserve or quota.
In Disk Utility, select an existing volume on the APFS drive and click the + (Add Volume) button in the toolbar, then name it and choose its format. It appears as a separate drive in Finder straight away, without erasing anything.
Unlike partitions, APFS volumes share the drive's free space, so you don't have to decide sizes in advance (you can set a reserve or quota if you want limits). That makes a separate volume a tidy way to keep, say, Time Machine backups apart from other files. See splitting one drive for Storage Studio and other uses.