How do I backup my iCloud photos to an external hard drive?

You'll have to do it in stages then. There is no direct download path to an external drive from iCloud.

Download what you can and then move that to your external drive.

See >>> https://support.apple.com/en-ca/HT204055#photos

There are two ways to increase iCloud storage space .

( a ) https://support.apple.com/en-in/HT201318

( b ) See some articles to reduce the data https://support.apple.com/en-in/HT204247

https://support.apple.com/en-in/guide/macbook-air/apdb468896e0/2019/mac/10.14.5

https://support.apple.com/en-in/HT206996

https://support.apple.com/en-in/guide/system-information/syspf5a64aa6/mac

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 https://support.apple.com/en-in/HT204264

And its preferences https://support.apple.com/en-in/guide/photos/pht5156cc968/4.0/mac/10.14 , 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 https://support.apple.com/en-in/guide/photos/pht6d60d10f/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

Read this : Back up the library in Photos on Mac - Apple Supportsupport.apple.com › en-in › guide › photos › Mac

You can try two things, depending on what you want to do.

One option would be to open Photos with the option/alt key held down and create a new system Library on the external. Once everything is downloaded, reopen Photos with the option/alt key held down, and switch back to the Library on your internal drive. Switch as required/desired.

or if you want to clear space on the internal drive.

One option to free up space would be to move your Photos Library to an external hard drive. Then quit/reopen Photos with the option/alt key held down and make the external hard drive file the system Library. Once you are sure all your photos are there, you can delete the Library from the internal drive if desired. Once you do this, the external hard drive will need to be attached to access/sync your photos.

Article if for iPhoto, but the basics are the same.

Photos library - Move your Library to an external storage device.       

Once you are sure all your photos are there, you can delete the Library from the internal drive if desired. Once you do this, the external hard drive will need to be attached to access/sync your photos.

There is one problem with this plan, as there is no easy way to be sure, that the download of the original files from iCloud to the library on the external volume has finished.

At least, I have not yet found any status message in photos that is showing if all originals have been download.

The status bar in Photos does not show, if really all the originals have been download. When we are shown "updated just now", we can only be sure that the adjustments and metadata have been synced, but not, if all originals are now stored in the library.

The only way I found, that I can be sure if a library is including all original image files, is to make a copy of the library and to open the copy in Photos (without turning on iCloud Photos for the copy). If some originals are still missing, we will either get a warning about incomplete items (macOS 10.1103 to macOS 10.14.6) or will be seeing some white thumbnails when browsing the All Photos album in Photos 5 on Catalina.

Or, if we duplicate the library with PowerPhotos, we will get a list of missing files, that could not be copied and need downloading from iCloud. I am not using "Optimise Mac Storage", and so all originals should automatically been downloaded to the Mac. My Mac has plenty of free storage (there are at least 500GB free and not just available), but still the Mac is dragging its feet about downloading photos from iCloud unless I force a download by working with the photos. There are always several hundred incomplete photos, when I check a copy of my iCloud Photos Library and I have to force the download of the missing items and copy the library again.

Have you found an easier way to check for originals, that need downloading from iCloud, Eric?

As it is currently, I cannot trust the backup copy of an iCloud Photos Library without checking for missing originals. I am always keeping separate backups of the original image files by exporting them, just to be sure I am having a copy of all originals.

Im trying to download my icloud data and photos to my ext.drive

need help thanks