Backing up the iCloud Photo Library when it won't fit on my internal drive I love the iCloud Photo Library. Formerly an Aperture user, I enjoy having my entire library available on every device and now use it as my master repository. I upload raw files into it, taken from my DSLR, and of course the photos my iPhone takes. It's great. None of the devices I have, carry sufficient storage to download the entire library and so I have them all set to 'optimize'. Which is fine for day to day use, and even while editing as it downloads when needed. But I keep reading horror stories about libraries getting wiped out through bugs in Apple's software, and just generally speaking am getting increasingly worried I have just 1 copy of my entire library: the copy in iCloud. 

So, I want to back up my library. But my drive is too small to make a copy of the library onto it. So, how?

"Attach an external drive" ... I tried doing this. And while I can store the library on the external drive, this then means I no longer have ANY photos on my laptop when I'm not at my desk.

"Create a second library with different sync settings" ... It's not possible to have 2 different libraries both sync down from iCloud, it turns out. 

What other options do I have?
Can you set your library to be on the external drive, turn on the setting to keep originals locally, then download everything.

Then take a copy of that library on the external drive, turn off the setting to download originals locally, and move the library back to the internal drive?

I can't see why that would not work ...