I moved my photos library and pics to a new disk. I now get "MISSING FILE"

Are you running a Managed or a Referenced Library?

A Managed Library, is the default setting, and Photos copies files into the Photos  Library when Importing. The files are then stored within the Library package

A Referenced Library is when Photos is NOT copying the files into the Photos Library when importing because you made a change at Photo -> Preferences -> General. (You unchecked the option to copy files into the Library on import) The files are then stored where ever you put them and not in the Library package. In this scenario you are responsible for the File Management.

You can set Photos to use a Library anywhere. You don't have to reference it.

The drive needs to be formatted Mac OS Extended or APFS.

Just create a New Library on the SSD, set it to copy, then merge the Libraries: Import photos from another library in Photos on Mac - Apple Support

If your old Library is in iCloud Photos, you'd have to turn that off to import into the new library.

I'm not sure when to turn iCloud Photos back on, but I would assume you could turn it on when you create the new Library and after setting that new Library as the System library in Photos Settings.

One limitation of having the Library on an external drive is you can't search it with Spotlight--I didn't even know that was possible. You can still search from within Photos.

I'm afraid you've stumbled onto one of the primary reasons for never using Photos in referenced mode. There is no way to relocate masters. The concept doesn't exist. Quite bluntly, if you want to use a referenced library my best advice is use some other app that has the tools for doing that. Photos doesn't. So, no there is no documented way to do what you want.

Sometimes it's enough to relink one image in a folder to get the rest, sometimes it has to be done one by one.

Can you restore things to the status quo ante? If you can then you can covert to a managed library, and moving is easy.

Barney, the OP has set up his library as a referenced library, which means the files are stored outside the library package, and this is the source of the issue. He can't restore from iCloud as the iCloud Photo Library does not work with referenced libraries.

max Hatfield wrote:

Thanks Yer_Man. You input is very useful and informative. Thank you very much.

When using a REFERENCE LIBRARY are the changes to the photo meta-data (ie location, notes about the pic etc) stored on the photo file or in the library file?

Library.

See if you can find something that writes the EXIF data. That would store with the file, but I don’t think it is very expansive.

https://www.exiftool.org/gui/articles/where_what.html

Anything that would store information with the file would be just as proprietary as Photos metadata.

As Barney says, the are stored in the Library but can be added to the file on Export - if you have a working Library, that is.

There are several, but how much do you want to pay? The best and most reliable is Lightroom Classic... approx $120 per year. It runs on Macs and Windows.

I said to do none of that. Turning on and of iCloud Photos is a supposition about how one may need to merge the Libraries.