I decided to stick to the safest approach to make copies of my backups. In other words, my Mac backs up everything to one external drive and then I copy that entire backup directory to the second external drive.
I researched and found this command to be good for any files to be moved and synchronized from source to destination directories:
rsync -av --delete /Volume/Drive1/MyPictures/ /Volume/Drive1/MyPictures
That worked pretty well for any directory I store on the external hard drive except for Mac's backup directories. Running this causes an error:
rsync -av --delete /Volume/Drive1/Backups.backupdb/ /Volume/Drive1/Backups.backupdb
failed: Operation not permitted (1)
rsync error: some files could not be transferred (code 23)
Any suggestions how to address this? I tried rsync -rtb and got the same error. Copying manually using Finder also alerts with The operation can’t be completed because backup items can’t be modified.
Best Answer
Funnily enough, starting with macOS 10.12 the Finder has been granted special powers to perform an 'exact copy' on Time Machine archives. Use the Finder and only the Finder to give your .backups new homes, because Finder will maintain not only all permissions and metadata but either maintain or translate inode references for each item in the backup.