Photos System Library on external drive bloating after saving Photos to iCloud

Just in case it helps anyone else. I am now monitoring iCloud photos and my external hard drive.

The Hard Drive grew to 1.12TB used out of 1.5TB but I am now getting space back rather than the file size growing - 377GB free and growing. In terms of uploads to iCloud Photos - I am at 1,937 out of 79,790 photos and growing at a rate of several a minute. It's certainly going to take days to complete the upload to iCloud but I am a bit more confident that the external Hard Drive will not get full up and stop the process. I still find it interesting that your computer or hard drive requires so much spare capacity before you can attempt this. I think there should be a warning before anyone turns on iCloud Photos. I assume but do not have conclusive evidence that the upload requires the Photos app to be open on the computer and that it doesn't happen in the background when it is not open. Can anyone confirm?

For anyone who was interested in my question. I can report slow but steady progress is being made with 32,000 out of 80,000 photos now uploaded to iCloud and 750 GB free and 750 GB system photos library on the external drive. Another 2 weeks or so and it will be done.

Once complete, with all of my photos on iCloud, I would like to move the system photo library from the noisy external 1.5TB drive where the library is currently back to a quieter 1TB portable drive (which was not big enough for the iCloud process to work) but how do I do that without deleting all of the photos from iCloud in the process? I'm sure there is both a correct way and a wrong way to do this. Can anyone tell me the correct way to proceed?

Thanks in advance.

Have now got to the point when I can try switching from one library to the other. Following the instructions, I re-opened Photos and selected to copied library. I then tried to make this the system photo library but got this message.

Switching the System Photo Library will turn off iCloud Photos.

Any photos and videos that have not been fully downloaded will be removed from this Mac and Shared Albums will be disabled. Turn on iCloud Photos for this new System Photo Library in Settings.

I'm not sure if it's OK to go ahead or if that will wipe photos either on my mac or in iCloud.

Currently, I can't eject the original photo library HDD as 'it's still in use'.

How should I proceed?

Thanks

John

Thank you. I'll try that way when the upload is complete.