Lightroom Classic has not been deleting photos from my Hard drive.

"Is there any way to identify all those images on my hard drive that are NOT in Lightroom so that I can delete them?"

1. If the ancestor folder (e.g. Pictures) containing all your pic folders isn't visible in the Library Folders panel, right-click one of the top-level folders and do Show Parent Folder. Repeat until the ancestor folder is showing.

2. In the Folders panel, right-click the ancestor folder and do Synchronize Folder. Select exactly these options and click Synchronize:

3. In the Import window, double-check that Add is selected at the top (it should be) and select File Handling > Build Previews: Minimal in the top-right corner (to make the import go much faster). If you're not seeing all the photos you think there should be, right-click the ancestor folder in the Import window's left column and ensure Include Subfolders is checked.

4. Click Import. 

5. When the import completes, select Previous Import in the Catalog panel in the upper-left.

6. Select all displayed photos and do the menu command Photo > Remove Photos, selecting the option Delete From Disk.

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I strongly recommend that you have verified your backups of both the catalog folder and your pictures folders are working before embarking on this.