Hi bro, I have to remind you that a RAW photo can't be edited. RAW is simply a sensor dump from the camera and cannot be edited. If you modify a RAW image, the changes can only be applied to a standard image format.
You can either export the unmodified original photos (RAW) or an image format (TIFF, PNG, or JPEG) and get the edited version.
Thus, you can transfer the RAW photos from your Mac to the external hard drive and then make modifications to these images on the external drive.
To move the RAW images to an external hard drive, you can open Photos on your Mac, select the photos you want to export, click File on the top menu bar, and select Export > Export Unmodified Originals.