How can I Mirror a Drive Without RAID

Quote from crashtest

rsync -av --delete /srv/dev-disk-by-label-EXT401/ /srv/dev-disk-by-label-EXT402/

If I enter folders behind the drive labels, does that also work to mirror certain folders? Or is it better to proceed differently?

Yes, you can. You can drill down to lower levels and go folder to folder. For example:

rsync -av /srv/dev-disk-by-label-EXT401/Music/ /srv/dev-disk-by-label-EXT402/Music/

The above would copy everything under "music" from the source to the destination.

You might want to leave out the --delete switch until you verify that the first transfer took place. Otherwise, if a mistake is made and the source is blank (a fat finger typo, etc.), the --delete switch will erase the destination.