Time Machine offers a powerful continuous archiving tool for files on your Mac. By design, Apple keeps it simple: it lets you select just a single set of volumes (and excluded volumes, folders, and files) on your Mac for all Time Machine backups, Time Machine can back up automatically only in one of three intervals (hourly, daily, or weekly), and whenever Time Machine volumes are plugged in, macOS rotates among them across the specified interval. (For instance, if you have Time Machine Volume A and Volume B and are set to hourly backups, macOS backs up to Volume A at 3 p.m., Volume B at 4 p.m., Volume A at 5 p.m., and so on.)
However, you can gain some control by mounting and unmounting volumes. For instance, say you have a 1TB internal drive and a 4TB external drive. You only mount the external drive from time to time. For backups, you have a 1TB SSD and a 6TB hard disk drive (HDD). You can’t back up 5GB to a 1TB drive.
The solution involves a little dance whenever you want to back up the 4TB drive.
Read more at Macworld.com