how to turn on iCloud in Photos preferences when iCloud says your mac does not have enough disk space to handle current Photos volume in iCloud. Do not want to optimize iMac....want to keep full resolution pics on iMac.

Hi

the process won't create duplicates. I can't say for certain that it will allow you to turn off optimise and will know there is enough space. That depends on the exact process photos uses to do that - which is not documented.

That method will create two copies.

Two possibilities that might work.

1 - turn on optimise storage, let the sync take place (That is inevitable - turnign off iCloud triggers a complete resysnc), then see if you have enough space to turn off optimise storage. However, this is a gamble as to whether you have enough space or not.

2 - Your other option is to completely move the library to external storage with the disadvantage that you must have the external storage connected whenever you want to use photos. (if you are now short of space you will probably have to do this eventually anyway.

The external storage must be formatted APFS or MacOS extended (journaled) (preferred for a mechanical drive), and 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 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)


See

https://support.apple.com/en-gb/HT201517

Cool - glad it's sorted.