Can't import photos to an external APFS SSD

You wrote:

"If my fix (rebuilding the library) has sorted my issue, to prevent it from happening again, is there a way to safely eject the SSD drive without manually stopping processes via Activity Monitor eg where a power failure causes a restart (or when I simply forget to)? Or do I simply have Photos closed/quit unless I’m actively using it?"

The external drive should always be mounted as long as you are signed into your user account, even if you are not using Photos. Connect the external drive, before you start up your computer and log into your account. And keep the drive mounted until you shut down your Mac or log out off your account.

Your System Photos Library needs always to be available for the background processes or other system apps trying to access the system photos library, or Photos may lose the connection to the system Photos Library. And this will cause exactly the kind of error you have seen. If the system can no longer access the System Photos Library it may switch to a new local library in your Pictures folder as a default.

So, what are you doing about the System Library? Do you change that when you make a different library active?

Have you tried using the app Image Capture that's in your Applications folder? This app seems more versatile than the built in routines, and many people have had more success by using that.

Quite often WD drives come with software on them to be installed. Did yours come with software and did you install it? If so it's not needed and should be uninstalled completely so as not to interfere with the system's disk management features.

Be sure to have the WD drive backed up to your Time Machine drive so that if you have problems with that drive you'll have a backup of the library to fall back on.

If you do have problems with the WD drive consider replacing it with one from Other World Computing (MacSales.com). I have 8 OWC SSDs connected to our iMacs via Disk Docks and SATA to USB adaptors:

These are not the fastest configurations (between 400-500 MB/s). In their own cases with the appropriate chip you can get 4 to 6 times the speed.

If you do consider upgrading the drive I recommend you contact OWC Customer Support to see which of the many configurations they offer would best fit your Mac model, workflow requirements and budget.

I don't keep my active Photos Library on an external drive, because I have a laptop, and to be mobile I would have to unplug the drive. There are lots of background processes going on, especially with Photos, so disconnecting the drive can cause complications. Since you use a Mac Mini, you ought to consider keeping the external drive connected. You may need a hub to offer enough ports.

If you switch between Libraries, you may need to decide how you will deal with the System Library-- the one that your Mac System thinks of when you need pictures-- like for Messages, Email, browser, and so on. Having Photos switch to a different Library doesn't change the System Library. That may affect importing pictures, as well. The System Library is set in Photos Settings>General.

I think you need to change SSD!

Good sleuthing! Hubs can be a problem with the Silicon Macs.

mgwil wrote: ... My issue is about restarting the Mini and potentially corrupting the library on shutdown if the SSD hasn’t been properly rejected. .... when a power failure occurs unexpectedly. Is there a way to handle that?

In normal circumstances, if the Mac is doing something that keeps it from shutting down properly, then it will tell you.

I have not had problems with power outages or surges-- I've always figured the devices could handle most of that, and for the usual things it seems so. A surge protector is a pretty good idea. I'm not sure about lightning strikes, but I have multiple backups, and that's one reason why. Most of my external drives that I use for backups do not stay plugged in, though one does.

The moral is, of course, keep backups.

Good to know, thanks.