Error Photos was unable to open the library “Photos Library.photoslibrary”. (3146) loading a photo library

Hi

Is the external drive correctly set up for using with 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 should not be on a memory stick, SD or similar.

It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt).

File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.

My guess is your drive is ExFat - which is not compatible with a photos library. Judging from the number of people with this error after upgrading to Big Sur 11.5 or later, MacOS is now refusing to open libraries on ExFAT drives - even if it has worked before.

See

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

OK - something is off.

I'm going to suggest starting in safe mode - trying the new library thing, then trying the external library. If that gets somewhere, restart normally, and try again.

Might also be worth trying creating a new user account on your mac and trying with that, just to check there isn't something odd in your normal account.

Also try a reset of NVRAM and SMC (If present on your mac)

https://www.macworld.com/article/224955/how-to-reset-a-macs-nvram-pram-and-smc.html

To be honest - if these don't get anywhere, you may need to contact apple support direct. (Link top right of this page). Many of the experienced users here may not have yet upgraded to Monterey (I haven't) - so if there are specific problems with that then it is difficult to help.

Oh - well that is interesting - rules out my theory somewhat.

Do you have space to copy it onto your system drive anyway? If so, I'd try that.

Are you able to create a new empty library, and open that?