I know that's a shocking headline, but that certainly seems to what they're saying for a certain group of users (red emphasis added):
By default, your System Photo Library is stored in the Pictures folder on your Mac, but you can move it to another location on your Mac or store it on an external storage device.
WARNING: If a Photos library is located on an external drive, don’t back up the drive using Time Machine. The permissions for your Photos library may conflict with those for the Time Machine backup
Jan 23 2018 update: Thanks to reader Brian for commenting below that Apple has updated this page with much clearer wording. It now reads (emphasis added):
If a Photos library is located on an external drive, don’t use Time Machine to store a backup on that external drive. The permissions for your Photos library may conflict with those for the Time Machine backup.
That just means you shouldn't use the same external drive for both your Photos library and as a destination drive in Time Machine. This makes much more sense; continue reading only if you care about my feelings on the original incorrect wording.
That's taken from the System Photo Library overview, part of Photos' help. In a nutshell, Apple recommends that if you've moved the System Photo Library to an external drive—as nearly anyone who takes lots of pictures will have done, given space-limited solid-state internal drives—you do not use Time Machine on that drive. Not just "don't back up the Photos Library folder with Time Machine," but "don't back up the entire drive with Time Machine." Yikes!
Think about that for a bit…this affects anyone with limited internal storage space who has their photos stored on an external drive. And in today's Mac world, that could be a lot of people—while you can configre some machines with up to 2TB of solid state storage (and iMacs with bigger Fusion drive), doing so is wildly expensive. So there are potentially a lot of Mac users with small internal drives who may be affected by this. Yikes again!
Apple's writeup leaves me with a couple of critical questions…
- What if I exclude the iPhoto Library folder from Time Machine—is that sufficient to prevent the permissions issues, such that I can use Time Machine for the rest of the drive?
- How, exactly, am I supposed to back up my photos, if I can't use Time Machine? (See update at end for Apple's recommended solution.) Will SuperDuper or CarbonCopyCloner also run into permissions problems? I'm using a 1TB Flickr account and their upload tool as a backup method, but I have lots of upstream bandwidth, so it's not bad…but not everyone is lucky enough to have fiber to the door.
I know Apple's answer to the second question is "You shouldn't be storing photos locally, they should all be in the cloud." But if you have a huge collection of photos and videos, and/or if you've got slow or limited internet, this is not a realistic option.
My library is over 40,000 photos and 1,400+ videos, requiring in excess of 500GB of storage. At that level, I'd need the 2TB iCloud plan at $10 a month…versus Google and Amazon, both of which offer unlimited photo storage space for free (though Google has caps on image and video resolution). So if I have to go to the cloud for primary photo storage, I don't think I'll be using Apple's solution (even though it's obviously the best-integrated).
Seriously, Apple, tell me how to back up the 8TB external drive I'm using to hold my photos…there must be an Apple-accepted solution, right?
Update: Ed Mechem's comment points out Apple's Back up thew Photos library page, which recommends simply dragging your Photos library to another drive to back it up. Thanks Ed; that appears to address the second question. I don't know if it's OK to copy it to the Time Machine drive, just outside the Time Machine folder, or if you'd ideally need a third drive. You'd want to use an app like CarbonCopyCloner or similar to automate this process, obviously. Manual backups aren't usually the best solution.