Photos won't open says 5% updating library
This is terrible, multiple libraries on a external back up and before last upgrade they opened promptly, need the images now from several libraries, WHAT has happened, I feel sick!
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Don't panic! When there is a major change in the macOS, the Photos libraries must be updated to take advantage of the new features in the Photos database. It takes longer that you want, but it should be over soon. Hang on.
If i doesn't make reasonable progress, come back here.