I take it that your goal is to add not already cataloged files from an old drive to your present catalog.
Important : first work on a duplicate of your catalog.
Locate the path of your present catalog via the menu Help / System info.
Copy the whole folder and paste it somewhere on your computer.
Open the organizer with this new catalog by double clicking on the catalog.pse12db (if you are on PSE12).
Now, keep in mind that the organizer will recognize duplicates by the 'date taken' plus the file size in Mb. Files with the same name and even slightly different file sizes won't be considered duplicates.
If you 'add' files from the old drive, most of the 'real' duplicates (according to above criteria) should be ignored. That's exactly what happened in a test I just ran.
The organizer can help you recognize the recently added files by the criteria 'import batch' which can be used to select or to sort files.
If there are imported duplicates with a difference in file size, they would show in thumbnail and date order view side by side with the already present files.
The fact that you can select or sort by import batch can help you 'undo' an import if it does not work as expected.