Photos library

Hi

Regarding the external drive - is it correctly formatted for a photos library?

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives)

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must have the 'ignore ownership" checkbox ticked.

It must be directly connected (No ethernet or wifi)

See

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

If formatted incorrectly (usually exFAT because that is the default for new drives) or not directly connected then photos is probably having to repair it on every open.

You may also want to check the external drive with Disk Utility, if it still working properly or starting to fail. Soon for later will each external drive start to fail and need replacing.

If the library on the external volume s your system photos Library and not just a secondary library, you should keep the external drive connected, whenever you are signed into your user account, even if you are not working with Photos. The system Photos Library needs to be available to the background processes at all times, so it can be updated, whenever the system needs access. Otherwise Hotos may lose the connection to the library and repair it, whenever you reconnect the drive.