Why isn't ownership preserved with "rsync -a" on my Mac?

Solution 1:

If you open the Disk Utility app and compare both drives, the internal drive will have Owners Enabled set to Yes meanwhile on the external drive it will be set to No.

enter image description here

Solution:

Open the Finder app, select the external drive, choose Get Info from the File menu, and uncheck the box ignore ownership on this volume.

enter image description here