Multiple partitions for SD! on external drive?
 

I'd like to make daily, weekly and monthly clones of my internal hard drive to an external one three times bigger.

Can I just partition the external drive into three partitions, and then schedule SD! to mount, copy and unmount the individual partitions as and when needed?

Sure. Although that means all your backups are dependent on one physical device not failing...

Good point!. Thanks.

How I did this
 

This enclosure worked well for this, connected to a 1TB SSD Mac Mini 2018 (now 95% full) through USB-C:
Mediasonic ProRaid USB-C 2 Bay 2.5” SATA SSD/Hard Drive Enclosure – USB 3.1 Gen-II 10Gbps Speed/USB Type C (HUR6-SU31C Grey)

I put a 1TB Samsung SSD in bay 1 (daily_backup) and a 2TB Samsung HD in bay 2 (containing weekly_backup and monthly_backup). I formated both as APFS. I left the enclosure's default settings for 'single disks' as is (no raid).

The initial SD! clone to the SSD ran at up to 250 MB/s and took less than an hour.

The second Smart Backup to the SSD was mostly over 1GB/s and took less than 20 minutes.

I use TinkerTool System to set the three drives to not mount on startup, and SuperDuper! respects that by automatically mounting/unmounting the volumes as needed. Nicely, there is no light on the enclosure when the drives are unmounted (but plugged in), and I can tell by the feel of it that the spinning drive is not spinning then. [More on this: the drive instructions say the USB port of your PC must support power-off function so that the device would go to sleep mode, so this seems to be what is happening with this Mac Mini].

Great! I'm glad it's working well for you.

Note that multi-bay devices sometimes won't boot drives in positions other than '0', basically. It's something you'll have to try.

Thank you, that seems to be true for this enclosure too. After I turn off security and allow external booting during a Command-R start, I can successfully select and boot from the external SSD in the enclosure (daily_backup). Perhaps by luck I may have installed it in the first position.

One of the spinning HD disk containers, the only one I have copied to so far, does show up in the Startup Disk utility that is accessible during a Command-R start, but if I select it as Startup Disk the machine boots from the internal SSD anyway.

Hopefully, if I am stuck, I can put the spinning HD into its own individual enclosure and boot from there. But to be honest, it is unusual for me to need to boot from weekly or monthly clones: most often I only mount them to grab a file I have accidentally changed or deleted, because it is quicker than using Time Machine. On the other hand, I often need to boot from the daily backup, so it is good to know that works.

By the way, my goal here is to replicate with the new Mini what I have used for ten years with a Mac Pro and its 4 internal drive bays. The multiple-drive external enclosure is not as robust as that, but for now it seems to be the best I can do.

You may also be able to swap the drives in that case...