SuperDuper!

Makes a startable copy of your Mac on an external disk, and can do it for free.

What it does

Copies a volume to another volume, in one pass, with a plain-English description of what it is about to do before it does it. Unlicensed it still makes full copies; a licence adds Smart Update, scheduling and unattended runs on your own Macs.

Smart Update is the feature worth paying for: it compares both sides and copies only differences, then deletes what no longer exists on the source. Minutes instead of hours, and far less written to the SSD.

Notes

Same platform limits as every cloner: the system volume goes through Apple’s asr, and a bootable result on Apple Silicon is not guaranteed.
No snapshot retention. A copy is the current state and nothing older, which is why it pairs with a versioned tool rather than replacing one.

Alternative to

Alternative	Type	Trade-off
Carbon Copy Cloner	Paid	Snapshot retention, richer scheduling, no free tier
Time Machine	Built in	Versioned history, but the result does not boot
`asr`	Built in	The replication engine both of these drive, with no interface at all
Clonezilla	Open source	Images any disk from a USB stick, but not the running Mac