Bacing up icloud photos to external disk
how do i backup original photos from icloud to external hard disk
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Do you have a Mac computer that has the Photos application?
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. Move your Photos library to save space on your Mac - Apple Support