How to maintain two full copies of a large photo library in iCloud and on iMac?

iCloud Photos is not a true backup service; it is a syncing service. If you delete a photo from your iMac, it will also be deleted from iCloud (and vice versa). To have two safe, independent copies, the library on your Mac must be fully downloaded so you can then copy it to an external hard drive.

1. Check the Photos app settings

Let's make sure the correct option is definitely selected:

 Open the Photos app on your iMac.

 In the menu bar at the top, select Photos > Settings (or Preferences on older macOS versions).

 Click on the iCloud tab.

 Make sure Download Originals to this Mac is checked.

2. Check the real-time download status

 Inside the Photos app, click on Library in the left sidebar.

 Select the All Photos tab.

 Scroll all the way down to the very bottom of your photos.

 Here you will see a line of text indicating the status. If it says "Paused" or "Downloading [X] items", your Mac is still downloading the full-resolution versions. If it is paused, click Resume.

3. How to force the download of originals

If the status says "Updated" but you are certain the original files are missing, you can force the Mac to re-verify the entire library:

 Close the Photos app.

 Press and hold the Option (⌥) + Command (⌘) keys on your keyboard, then open the Photos app.

 A window will appear asking you to start the Library Repair. Click Repair.

 This process will re-analyze the library, fix any discrepancies, and force your Mac to download any missing full-resolution originals from iCloud.

4. Creating the actual "Second Copy" (The Real Backup)

Once the download of the originals onto your Mac is complete (you will know when it says so at the bottom of the "All Photos" tab), you will have one copy synced in iCloud and one full copy on your iMac.

To turn this into a truly secure, independent backup:

 Close the Photos app.

 Open Finder and go to your Pictures folder.

 Grab your Photos Library file (⁠Photos Library.photoslibrary⁠) and copy it manually to a spacious external hard drive.

By doing this, you will have one synced copy in the cloud, one working copy on your Mac, and a safe, static backup copy on your external hard drive.

You have told us what you’re trying to accomplish: You want a backup copy of your Mac’s large photo library.

You have also told us how you plan to accomplish this: You want to save your photos in iCloud.

But there are certainly better ways to accomplish your objective!

iCloud is not intended to be a backup source for your Mac. That’s what Time Machine does. iCloud is a place to keep data you wish to share among your various Apple devices.

Use Time Machine. It’s designed to accomplish backups.

Back up your Mac with Time Machine - Apple Support

KateEU wrote: …why has the collection been uploaded though - leaving only “ smaller “ files on my machine ?

As you know, what you describe is not normal. Not being there at the time to watch, no one here can know what was done to cause this.

There is so much back& forth here that I can't follow what you've done. I understand that you have a good copy of your Photos Library on an external drive. So, if I were you, I would erase the bad version that you have in the Pictures folder, and then I'd drag that good copy into its place in Pictures on your Mac. Then I'd make that new Pictures Library the System Library in Photos' Settings>General.

Then I'd sit back and wait. All the pictures in this newly made System Library would be copied to iCloud Photos, with Photos avoiding duplicates. It could take awhile. I would eave the computer on, awake, and with Photos in the background at night. I would want it available but not thinking that I'm using the Photos app.

It is true that iCloud Photos is only a limited sort of backup. That's also true for active files in iCloud Drive. But you can use iCloud Drive as an additional backup to your external drive backups. As I said earlier, to put a Photos Library on iCloud Drive, you can remove the ".photoslibrary" extension to let iCloud Drive know that you're not trying to actually use the package as an active Library. Alternatively, you can zip the folder or use a DMG "Disk Image" of the created with Disk Utility. For a DMG file, the ".photoslibrary" extension must be removed.

The Library Repair process is the perfect solution for this: it will force macOS to re-verify the status of every single file and finally trigger the download of all your full-resolution originals. Let me know how the repair goes!

It's not clear to me what you are doing. Maybe that's because you say iCloud, but not iCloud Photos or iCloud Drive, etc, so I can't tell what you mean--the are such different things. You can not have two iCloud Photos Libraries on one Apple Account. Or are you saying that you have two accounts? And are you saying you want to save a Photos Library, or is it copies of pictures from a Photos Library?

Optimize Storage certainly allows you to keep only 20% or less of your Photos Library locally. A picture does not have its Original file stored locally can't be edited until the original is downloaded from iCloud.com. Do you see

"downloading" when you edit pictures? (this is from an iPhone, but it's similar on a Mac.)

You can't move a Photos Library to iCloud Drive-- it won't be accepted, since it can't work from there. Here's what iCloud Drive looks like if you try:

However, if you remove the ".photoslirbrary" extension, then it will be accepted as a regular folder. Alternatively, you can make it a zip file or a DMG file.

Can you tell us, do you see all of your pictures in iCloud Photos at iCloud.com?

As sberman says, keeping a copy of your Photos Library on line somewhere does not mean that you don't need to keep a copy on a separate drive at home.

Let us know some details so we can help…

KateEU wrote:

On the advice given i will try another storage option if this plan is not advised but how do I start again ?

I don’t really know what this means.

• “… if this plan is not advised …”
  • What plan exactly?
  • The plan to rely on a Time Machine backup is absolutely advised.
• “… but how do I start again?” Details are provided in the article I linked above. And, of course, be certain your photo library on your Mac is stored locally exactly the way you want it.

This may also help regarding restoring full size photos:

Download photos to your Mac from iCloud - Apple Support