Hello munsl,
Thank you for using Apple Support Communities.
Based on your post, I see that you are trying to move your Photos library to your external WD My Passport drive. It appears to be reducing the file size, and you want to transfer the original file size and quality. I can imagine that it's concerning to not be transferring over the Photos Library how you want. I can help assist you with this.
Please use the article Photos for OS X: Back up your Photos library, specifically, this section:
Even if you use iCloud Photo Library, it’s important that you always back up your library locally using one of the following methods:
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Use Time Machine: After you set up Time Machine, it automatically backs up the files on your Mac. If you ever lose the files in your Photos library, you can restore them from the Time Machine backup. For more information about setting up Time Machine, open Time Machine in the Applications folder, then look in the Help menu.
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Manually copy your library to an external storage device: Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy.
If you have more than one photo library, be sure to back them all up.
Important: If any of your image files are stored outside your photo library, those files (known as referenced files) aren’t backed up when you back up your library. Be sure to back up these files separately. To make it easier to back up all your image files at once, you can consolidate referenced files into your library. For more information, see Change where Photos stores your files.
If you are manually moving from your Mac to an external drive, make sure that you are dragging the Photos library to the external drive to create a copy.
Best Regards.