iCloud Photo Library, when it fits your needs, is a great way to avoid having to manage where your images and videos wind up. You capture video on your iPhone or drag an image into Photos in macOS, and it just syncs everywhere while making a central copy at iCloud. While I hear regularly from people having difficulty with aspects of it, it's a way to reduce the stress about how much storage you have on any given device, especially iOS devices.
(And, yes, once again: if you delete an image on one device that's linked to iCloud Photo Library, it deletes it everywhere. Apple's warning when you try to delete is real.)
However, there's one configuration I can't advise, and Macworld reader Eric writes in with a question that prompts a discussion. He's wondering if he could rely on iCloud to be his "main backup of images." The short answer is no, but it's not about distrust in Apple's technical abilities. Rather, about the frailty of all material things, and the risk of putting all one's digital eggs in one basket, no matter how firmly the basket-storing company is holding that basket.
iCloud Photo Library can be configured in iOS and macOS to either retain media you capture or import on those devices and download at full resolution any photos and videos you import and capture on any other linked computer, phone, or tablet. However, as I've written about many times, the optimized storage option is best for most people on their iOS devices, because most of us with sufficiently large photo libraries lack the storage necessary on an iPhone or iPad. (I paid the premium on my latest iPhone to get a 256GB—but my Photos library is over 230GB.)
You can also enable this in Photos for macOS (Photos > Preferences > iCloud), but if you enable this feature on all linked Macs (whether that's one or more), you no longer have a full-resolution copy of all your media. Your local copy of Photos reliably retains only images and videos you've viewed or that it hasn't shifted to a thumbnail only storage. There's no way to control which media remains locally cached at full resolution.
As a result, you're relying entirely on Apple for this backup, and you can't reach out to iCloud and backup that backup. It's a single copy, which means a single point of failure. While Apple has been very reliable and, like all cloud providers, uses multiple geographically redundant storage coupled with forms of deep storage and backup, we don't know the details about this nor under what circumstances a failure could occur. (Some cloud providers are more specific about where data centers are and their backup and geographical redundancy.)
This is why all good backup advice recommends you have effectively three copies of a given thing you want to retain forever: a live copy on media you can access; a local copy that's offline or physically and mechanically separate; and an off-site copy that you could retrieve. People recommend different combinations of this and different strategies. (This is often called "3-2-1": three copies, two local, one off site.)
My configuration is:
- The Photos Library resides on an external drive.
- That drive is backed up continuously to a cloud-based provider (Backblaze).
- I have iCloud Photo Library enabled with full-resolution storage on my Mac.
I should probably also routinely clone this media to a drive I stick in a safe-deposit box, to keep it safe in case of fire, flood, or natural disaster hitting my home, but I haven't pulled that quite off yet. (A 4-2-2 solution, I suppose!)
Eric also asks if Time Machine copies high-resolution or thumbnails when you have Photos in macOS set to use optimized storage: Time Machine only copies data that's stored locally, so any images or videos that exist only as thumbnail views locally will be backed up in that fashion.
Conceivably, Apple could create a kind of high-level iCloud API that it could offer to other cloud-hosted providers to use with details you provide, so you could perform additional, non-Apple backups off-site encrypted backups. I can't imagine the company doing that, because that sort of behavior signals to users that the service is less reliable, when it would in fact offer more reliability on top of what seems to be completely solid reliability.
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