How to use the Photos library repair tool AND specify WHICH library to repair

You have to select the library you want to repair: Try it this way:

• Quit Photos, if it is running.
• Make a current backup copy of the Photos Library, if your backup is not current. Do not overwrite an older backup of the library, from the time the library has been still working. You may need it.
• Open the folder with the library to be repaired in the Finder
• Hold down the key combination ⌥⌘ while double clicking the library to be prepared - do not click the Photos.app
• Photos will now open and repair the selected library.

This will only work, however, if the the Photos Library is on volume, that is compatible with Photos. You must not try to repair the library on a volume that has been used for Time Machine backups, or is accessed over the network. The file system format should be MacOS Extended (Journaled) or APFS (for Mojave or Catalina).

You may want to give Photos full disk access in System preferences > Security & Privacy.

Please check, if your external is compatible with all the requirements listed here:

• Move your Photos library to save space on your Mac - Apple Support

Read/write access alone does not suffice. Catalina has more strict security features than the previous versions.

• Your external volume must not have been used for Time Machine backups of must not have any backup software installed.
• The file system format needs to be MacOS Extended (Journaled) orAPFS, not case-sensitive
• It must not be a network volume, but locally mounted
• And it must remain mounted and plugged in as long as you are signed into your user account, even if you are not working with Photos,

Paul, where is your Photos Library stored? And which system version is running on your mac? If Photos cannot open the repaired library after you quit Photos, there must be a problem with the location, where the library is stored, probably a permissions error.

ps - I was not sure what this bit meant in practical terms "And it must remain mounted and plugged in as long as you are signed into your user account, even if you are not working with Photos, " please clarify

The problem is, that there are many system processes, that are working in the background with yor Photos Library. even if you are currently not working with Photos. The system will look for your System Photos Library, as soon as you log into your user account. If the volume with your Photos Library is not available, because you did not connect it, Photos may consider your Photos library to be missing and ask you again to select a Photos Library the next time you launch Photos.

No, not a problem. Journaled means that the operating system keeps a continuous log (journal) of the changes made to the files on the volume. Makes it a bit easier to recover files, etc. I believe you can turn on journlaling without having to reformat the drive. Check to see if you can.

This procedure is now available as a new user tip: How to Repair a Specific Photos Library i… - Apple Community

The EHD should be formatted as shown in this screenshot:

Is it?

You're welcome 😊

Stay safe as well!