Is there a command to scan a disk from the Terminal.app?

You can use a standard “open” command to scan a disk or folder.

Here is an example shell script to scan a disk which mounted on “/Volumes/Example Disk”. The disk would be scanned and added to the primary catalog file. (You can change the primary catalog file (first startup catalog file) at the General preferences panel of DiskCatalogMaker.)

open -a /Applications/DiskCatalogMaker.app '/Volumes/Example Disk'

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