ChronoSync

ChronoSync is for the case where dragging folders in Finder has stopped being good enough and Time Machine was never the right shape. You define a job — this folder to that drive, bidirectional, skip these extensions, archive anything it replaces, run every night at eleven and also whenever the drive appears — and then you stop thinking about it. Targets can be a local disk, a network share, an SFTP server, an S3-compatible bucket or another Mac. It will also clone a volume, so it doubles as a backup tool for people who already own it.

The licence is the reason it sits high on this site. Econ Technologies write on their own product page that they never charge for new versions, and they have behaved that way across many years of releases. That is rare in this category, where paid major upgrades are the norm.

Two things to price in. ChronoAgent, which makes Mac-to-Mac syncing fast and reliable, is a separate purchase per remote machine. And the store quotes "From: $49.99", so check the seat count before you assume one licence covers the household.

The interface will not charm you. If you want one folder mirrored to one drive and nothing else, a simpler tool or a rsync line is less work than learning this.