NO. Do NOT start a new catalog.
Do NOT Import anything to a new catalog
Do NOT try to Synchronize any folders in Lightroom.
The correct procedure is to open your Lightroom Catalog that is on your Mac,
Connect the HDD that is the backup copy. If it is an 'exact mirrored' copy (of the dead HDD), and with the same Drive name, it may automatically re-link your photos to the catalog.
Or- Follow all the steps in this linked article to Re-link the folders that will show a question mark [?] to the backup copies of your photos on the backup HDD-
Thank you for the suggestion and sharing the link. I will try it.
I followed your suggestion,
"Connect the HDD that is the backup copy. If it is an 'exact mirrored' copy (of the dead HDD), and with the same Drive name, it may automatically re-link your photos to the catalog."
and it worked. Thank you very much!
I am pleased it worked.
Sometimes a brief answer can be confusing, hence my link to a detailed article.
Now you MUST immediately make another HDD backup of the image files, and keep up frequent catalog backups.
Good advice is that you need a minimum THREE COPIES of your photos to be able to say you 'have a backup'.
Restoring a Catalog links to images is one reason I like to keep one of my image backup HDDs as a 'mirrored' exact copy.
Three copies sounds like a good idea. I must do it lest I am all tensed when something like this happens again. Thank you very much for your valuable suggestion.