Move Photos library to external drive while maintaining iCloud and Time Machine

First, we have to ask, how is your external drive formatted? To avoid damaging the Photos Library, an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac           

If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

iCloud doesn't care if the System Library is on an external drive. Since you're using a desktop Mac, that's a very reasonable thing to do. There might be a warning when you change the System Library that your previous drive will no longer be kept in iCloud. That might be what you saw. You would just drag the Photos Library from the Pictures folder to the external drive. You can run it by double clicking-- then it will be the default.

The Time Machine backup will include external drives unless you exclude them, or if they have Time Machine backups on them. I'm not sure if Time Machine backs up ExFAT drives.

Do you use Optimize Storage? This can reduce the size of the Photos Library to maybe 30% or even less. However, if you do lots of editing, though, this may not be the best thing.

Another way to save storage is to divide the Library in two (or more) pieces. I have a smaller (150GB) Favorites Library that I keep as the System Library on my unoptimized Mac, and the Archive Library can be stored on external drives. I actually have more Libraries than that. You can see this User's Tip from léonie,

How to Split a Photos Library and Preserv… - Apple Community

One more thing, Rob - it is wasteful to move a photolibrary, that is syncing with iCloud. When you move a library to a new location it will no longer be syncing with iCloud until you enable iCloud Photos again. iCloud Photos will treat it as a new library and upload all items from the moved library again into iCloud and merge them into the existing library. This will take a long time and is a needless overhead. And you may nor even be able to enable iCloud Photos for this copy, if you do not have enough free iCloud storage to hold a second version of your current Photos Library. iCloud will estimate the required storage using the worst-case assumption that all photos are different from the ones already in iCloud.

If your current Photos Library is already in full sync with iCloud Photos, it would be much faster to create a new, empty library on your external drive (prepare this drive as described by Richard).

Then make the new library your system Photos Library in the Photos > Settings and enable iCloud Photos for this library. Your current iCloud Photos Library will download to the new empty library and you will save the long wait during the initial new upload and merging. It will be much faster.

But there will be two draw-backs:

• Some of the named faces may be missing, because not all named faces may be coming back from iCloud.
• The projects are not synced with iCloud, so your slideshows and books cannot be restored from iCloud.

You may want to keep a copy of your current library on the external drive as well, to save the projects. But do not sync this copy with iCloud Photos.

After starting over with an empty library it will help to use "Optimize Storage" during the initial download, so you will quickly get access to all photos, but then switch back to "Download originals", so Time Machine can back up your original images.

1) When you plug in an external drive, that drive appears in the Exclusion list.

Just remove it with the - sign.

2) Optimize is pretty transparent. You don't notice it if you just look at pictures. If you want to edit or crop, you may have to wait a moment (not long) for a download.

Optimize is especially useful on iPhones that have limited space. On my iPhone, the Photos Library takes up about 6% of the total Library size. I don't notice it at all scanning through pictures, but editing keeps more of the files local.

An optimized Library takes up more space on the Mac, because it has to accommodate a larger screen.

My comment about a desktop was because, with a laptop, we would likely unplug the drive a lot. That can be very confusing for an app like Photos that depends on background processes. With an iMac, you would keep it plugged in, so it's not a problem.

Urokguy wrote:

1) If I understand you correctly, the Photo Library on my external SSD will automatically sync with iCloud and the photos in it will be backed up by Time Machine without me doing any additional steps. Is that correct?

2) I do not use Optimize Storage and want to try it. ...

There is considerable doubt about how thoroughly Time Machine can back up a Photos Library that is using Optimize Mac Storage. Time Machine will not have access to the full resolution versions of any photos or videos that have actually had their storage optimized. If you eventually need to use your backup copy to recover the library to a specific point in time, you may find that you are dependent on whether iCloud copy still contains all the items you need in full resolution.