Copy iCloud Photos library to a clean install I'm doing a clean install on a new HD for various reasons, and I'd like to move my 700GB Photos library over. I also want the library to continue to sync with iCloud.

Since the existing library is synced, one way would be to create a new empty Photos library in the new system, turn on "iCloud Photos" and let it download the whole set from iCloud. Unfortunately that would take weeks and exhaust my data cap many times over, so I'd like to avoid this if possible.

I don't think I can use Migration Assistant because it will also move over all the stuff I'm doing a clean install to avoid.

I tried copying the system Photos Library, "Photos Library.photoslibrary" to the new HD. The copy went fine, I booted from the new drive and opened it, set it as the system library, and turned on iCloud Photos. It said "Updating" for a minute, then switched to "Uploading 100,000 photos". I paused it immediately and turned off iCloud Photos. I'm assuming this means it would treat all the photos in the copied library as new, and duplicate everything.

So my question is, is there a way to copy my full Photos library and maintain iCloud sync without downloading all 700GB from iCloud?
I ended up following ankii's suggestion and just chancing it by turning iCloud Photos back on and letting it upload 100,000+ photos.

On the plus side, it did sync successfully without duplicating any photos.

On the minus side, I started it on 09/08 and it just now finished on 09/17. So that's 9 solid days of "uploading". It was on a mostly idle iMac with broadband and it didn't seem to be taxing either the CPU or the network, so I'm not sure why it went so very very slowly.

My total data usage for the time is less than half the size of the library, so it does appear to have saved some data. No way to tell how long it would have taken to download the library from scratch.

I still feel like there should be a better way, but I can report that copying the library to the new system and turning on iCloud Photos does work.