How can I free up space on my MacBook Pro without losing iCloud Photos?

iCloud Photos is a synchronization service – not an archival one.

If you are synchronizing photos with iCloud Photos, deleting them from any device connected to iCloud Photos – whether that is an iPhone, iPad, or Mac – is a request to delete them everywhere, including in iCloud.

What you are asking for is not possible - and is not how the system is designed to work.

You can go into Photos > Settings… and turn on Optimize Mac Storage. That would give your Mac permission to substitute lower-quality, space-saving copies of your photos and videos for full-size copies when it thinks space is getting low. The decision as to when to replace full-size copies with space-saving copies – or vice versa – is under the Mac's control; this does not give you manual control.

I keep my Mac set to "Download Originals to this Mac" because that way, all of the local backups that I make of the drive that contains my Photos library contain full-size copies of my photos and videos.

TheLittles wrote:

"'if you … remove items, the changes appear on all your devices'"

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I'm mentioning about deleting from your computer --not iCloud. iCloud is its own storage --as is your computer. So, back these up, and they'll stay on iCloud.

No, they won't!!!

If the system Photos Library on your Mac is synchronized to iCloud Photos, and you delete photos out of it, you will be deleting the photos everywhere, including from iCloud.

My system Photos Library is synchronized with iCloud Photos. I tried deleting a photo from it. Photos put up a warning that if I continued, the photo would be deleted on all of my devices.

Oops-- there seems to be confusion, here. iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept just the same as the iCloud Photos Library. So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture from Photos on your Mac, then that picture is deleted at iCloud Photos and on all the other devices. 

You can store pictures in iCloud without having them in iCloud Photos-- by putting them in iCloud Drive.If you do that, then you can still see them on devices connected to iCloud Drive, but you can't edit them or add comments, etc. Also, you have to decide if you want to store the unmodified original files, or if you want to keep edits and comments. You may choose to save both, taking up twice the storage space. You would put pictures in iCloud Drive, or anywhere else, by using Photos' File>Export function. Then if you delete these pictures from your Mac Photos, they will be deleted from iCloud Photos, but the files will remain wherever you exported them.

A Photos Library has way more data than just the pictures, so exported individual picture files lose valuable information. You cannot use or even store an actual Photos Library in iCloud Drive-- but you can store picture files there.

You didn't say why you are using iCloud Photos. If you are not in need of synchronization of your Photos Libraries on your Mac and on your iPhone, then iCloud may not be useful to you at all.

The best backup, for instance, is to use Time Machine on an external drive. For just pictures, the best backup is to drag your Photos Library to an external drive, making a copy of it there.

We can help with details…

"Increasing data storage on macpro: How can I delete the files in my Photo App without deleting them in iCloud Photo in order to increase my data storage on my MacPro? Thanks"

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Export, then Delete from Photos:

If they are uploaded to iCloud, then they will not be lost when deleted from your Mac. You should get an external hard drive or flash drive, and then export them to it. From there, perform the transfer, and then trash what is in your Photos app; all will stay in iCloud. See here: Move your Photos Library to Save Space on your Mac - Apple Support

TheLittles wrote:

"Increasing data storage on macpro: How can I delete the files in my Photo App without deleting them in iCloud Photo in order to increase my data storage on my MacPro? Thanks"

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Export, then Delete from Photos:

If they are uploaded to iCloud, then they will not be lost when deleted from your Mac.

Use iCloud Photos on Mac - Apple Support

"if you … remove items, the changes appear on all your devices"

"'if you … remove items, the changes appear on all your devices'"

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I'm mentioning about deleting from your computer --not iCloud. iCloud is its own storage --as is your computer. So, back these up, and they'll stay on iCloud.

“No, they won't!!!“

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And that’s why you export them, before deleting.