How do I download all full res photos from iCloud to external hard drive?

Photos can’t be transferred directly from iCloud to an external hard drive for that you need to set up iCloud photo library see the article Set up and use iCloud Photos - Apple Supporthttps://support.apple.com › en-in

And its preferences Change preferences in Photos on Mac - Apple Supporthttps://support.apple.com › en-in › guide › photos › mac  open finder > Go > home > your user name > pictures either drag and drop Photos Library.photoslibrary to an external hard drive , the second method is right click on Photos Library.photoslibrary and click on show package contents in the upcoming column , you can also drag and drop complete masters folder to an external hard drive that is connected .

Also as per the article Back up the library in Photos on Mac - Apple Supporthttps://support.apple.com › en-in › guide › photos › mac

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.See this article also

Photos library - Move your Library to an external storage device

When photo library and its preferences are set up , the photos are not selected from iCloud.com or photo application and downloaded in Mac , so they will not occupy Macintosh HD space ( what you called as local storage ) they will occupy iCloud storage space

The photos are just synced with iCloud , if they are having less space in Macintosh HD , go to finder >applications > delete unknown third party apps , empty the download folder ( .dmg files ) .

See the screen shot click on guest > on the right arrow if they found any files , their home folder > click on right arrow , check any files / folder created in subfolders like music , pictures , movies > public .

Another is shared folder , check what data is still showing , in the second column , is there any folder created with some files , they are to be checked as they come under category for occupying Macintosh HD space .

See this article How to free up storage space on your Mac - Apple Support

Make sure you're signed into your computer with the Apple ID the 100k photos are in.

Go to System preferences>iCloud> and turn on Photos.

1. In the Photos app on your Mac, choose Photos > Preferences, then click iCloud.
2. Select the iCloud Photos checkbox.
3. Select Optimize Mac Storage: Stores smaller versions of your photos on your Mac when storage space is limited, and keeps the original, full-size photos in iCloud.
4. Once all your photos show, try the export process again.

Try opening your Photos library on your Mac, select all your photos, then choose File>Export> Export...items>Export, then choose your external drive as a destination.

Seeing as how I can't try this myself, I'm not sure. I think it should export high-res. You could try running one test photo.

Don't all of those solutions result in low-res photos being saved, still relying on the cloud for storage of full-res originals?

In photo application , open its preferences > iCloud section check mark download originals to this Mac .

but 4flutterby doesn't have enough space on the Mac to store 100k photos.

They can upgrade the storage plan .

Downloading originals to the Mac will take up local storage space the user doesn't have (as mentioned in the original question) no matter how much iCloud you have.