How can I backup photos to an external hard drive
How can I backup or transfer to an external hard drive
iMac 27″, macOS 10.13
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If you just want a backup, then use time machine.
If you want to transfer your library, then:
You can completely move the library to external storage with the disadvantage that you must have the external storage connected whenever you want to use photos. (in fact you should have it connected whenever you log in) You can then still use the photos app to view, manage and edit your photos.
To correctly prepare the external drive:
It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives)
It Must not be (or have been) used for time machine.
It must have the 'ignore ownership" checkbox ticked.
It should not be on a memory stick, SD or similar.
It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt).
File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.
It stands to reason that if you want any sort of reasonable performance it should be a fast drive with a fast connection.
It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt). File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.
See
https://support.apple.com/en-gb/HT201517