Where is it safe to store a Photos Library? Requirements for the external drive
This user tip has been moved to this new location: Where is it safe to store a Photos Librar… - Apple Community
The old version will no longer be updated, because editing it will change the formatting.
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Photo Libraries tend to grow over the years, and sooner or later they will be too large to be stored on the system drive. We can move our Photos Libraries to an external drive, but the drive needs to be correctly formatted as MacOS Extended (Journaled) or APFS and locally mounted. Do not format the external drive for the Photos Library as case-sensitive, if the internal drive is not case-sensitive. A case-sensitive drive for the Photos Library can cause data-loss, because files may not be found. Videos and life-Photos may not be able to access the video component. And this will happen even if the system drive is also case-sensitive.
And do not put the library into a synced storage, like Google Drive, Dropbox, or iCloud Drive. Also, the drive must not be read-only, even if you just want to read the library and not edit the photos (If Photos can't create, upgrade, or open your library because of file system issues - Apple Support).
Apple's support documents have been a bit vague about the requirements for a drive to host the Photos Library.It is not even clear, if Photos can work with a file system, that is not HFS+: This restriction is only mentioned for system Photos Libraries, that sync with iCloud - an iCloud Photo Library needs to be on a drive formatted MacOS Extended (Journaled)(https://help.apple.com/photos/mac/1.2/?lang=en#/pht211de786)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. However, to use iCloud services, the external storage device must be formatted using Mac OS Extended (Journaled) format, also known as HFS+.But it is clear from experiments when migrating libraries, that Photos cannot migrate iPhoto Libraries or Aperture Libraries to a Photos Library, if the file system is not HFS+. It will try to create the migrated library on the system drive instead (probably because it cannot create hard links on the drive) and usually fail by running out of storage.
And Apple confirmed now, that it is not safe to store the Photos Library in a synced Cloud storage, see: Updating from iPhoto to Photos for OS X - Apple Support
If you store your Photos library in a location that's part of a cloud service (for example, Dropbox, Box, or Google Drive), your library might be damaged by the syncing process. As a result, you could lose photos.To see your photos on all your devices, you can use iCloud Photo Library. To back up your library, you can use Time Machine.And Apple advises against storing the Photos Library on a network share as well, just like previously for iPhoto Libraries and Aperture Libraries.This is documented in: Move your Photos library to save space on your Mac - Apple SupportYou can store your library on an external storage device, such as a USB, FireWire, or Thunderbolt drive formatted as MacOS Extended (Journaled). Apple doesn't recommend storing photo libraries on external storage devices like SD cards and USB flash drives, or drives that are shared on a network.You can't store your library on a disk that's used for Time Machine backups.The previous version: Sharing photo libraries among multiple users - Apple Support
Connect an external USB, FireWire, or Thunderbolt drive which has enough free space to contain the library.
Be sure to choose a locally mounted drive. Storing your library on a network share can lead to poor performance, data corruption, or data loss.The linke document also explains, how to format an external drive to be used for Photos Libraries and to set the "Ignore Ownership" flag, if the drive is to be used from different user accounts or computers.
You should not use the drive you are using for Time Machine to host your Photos Library, or Time Machine cannot back up the library. See: Back up the Photos library on Mac - Apple SupportWARNING: 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.Occasionally Photos will also not be able to connect to the library again, if you want to open it a second time.
This user tip was generated from the following discussion: Where is it save to store a Photos Library? Requirements for the external driveApple fixed the Photos Help. The paragraph below is no longer valid:
Photos 3.0 on macOS 10.13 High Sierra introduced an added complication. If we move a library to an external drive, the library can no longer be backed up using Time Machine. We have to use a different backup method for the library, for example clone the external drive to a backup drive.https://support.apple.com/guide/photos/system-photo-library-overview-pht211de786
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.