Unable to preview .CR2 icons on External Hard Drive. View options are set to show options.

Right so it is an NTFS formatted disk, now Macs can happily read them but not write to them, unless you use third party software, or other methods, are you using anything like that.

How did you get the images on to the disk, did you use the Mac.

Is the WD passport new, the trick when buying a new external hard drive is to connect it to the Mac, open Disk Utility

and erase it and format it either Mac OS Extended (Journaled on a GUID partition scheme, or if you are going to be sharing it between Macs and Windows machines then choose exFAT on a Master Boot Record partition scheme, exFat because the size of files is not limited. When Disk Utility is opened make sure Show All Devices has been selected from View in the menubar.

highlight the Disk not the Volume, and perform the Erase/ format.

Before you decide to format your drive you will need to back up all the files that are currently on it as they will be erased during formatting.

Also when buying disks from WD or Seagate or others they may have their own software included, for Macs it is best not to install it just format the disk as mentioned.

Are these raw images all taken on the same Canon camera, can you open the files to see the image. Is it possible that the images were taken as a mixture of uncompressed and compressed raw settings, with some cameras the Apple RAW engine can only read uncompressed raw images. If you drag the non previewed images to your desktop on your internal drive, are the icons previewed there. Are all the images visible in camera. Have you run Disk Utility> First Aid on the external drive, to check for possible errors.

How much time have you given for the previews to generate, sometimes it can take a long time, any more information you can

give us might help.

You can try booting into Safe Mode then booting normally which cleans some caches.

Use safe mode to isolate issues with your Mac - Apple Support

look here:

Convert-cr2-Images-to-JPEG-on-a-Mac

Oh and just another thing have you tried unchecking and then re checking the Show icon preview button, on the odd occasion the my Mac randomly does show icon preview this has sorted it out.

What is the external disks file system.

What NTFS driver are you using to write to an NTFS drive and is it up to date?

FWIW, Seagate's Paragon driver 15.4.19 hasn't been updated since Oct. 2nd, 2018. Paragon's paid version is up to 15.5.41 and was updated April 29th 2019. It may or may not be the problem.