How can I mirror a folder to an external SSD?

When you say "mirror" do you mean in the traditional sense of real time changes? Changes you make in the original show up in the "copy" immediately? If you do, you'll need something other than a traditional backup solution. Time Machine, Carbon Copy Cloner, SuperDuper! make snapshots in time. But until the time delay is satisfied, nothing is "mirrored" i.e. copied to the folder.

The copied folder will not be updated "on the fly" ... but at specified time intervals. During those intervals, nothing is updated.

Balu_99 wrote:

I don't need real time changes, sorry for the confusion! I just want to synchronize the given folder on the external SSD.

"I just want to synchronize the given folder on the external SSD." Synchronize means real time changes.

When you make a change to a file in the folder then that change is reflected in the synchronized folder on the external device.

Try Carbon Copy Cloner. It can clone a selected folder elsewhere, and subsequent clones are fast.