question mark

Yes, adding a second external hard drive could be the problem.  When you connect the computer with the drive where the photos are stored, it must have the same drive letter designation as when the files were originally imported into the catalog.  I have no idea how Macs assign the drive letters. So perhaps Jeff can weigh in about that.

A secondary issue that can occur with Windows is that two identical hard drives can be given the same internal ID number and this can confuse the Organizer.  I have never seen that being a problem with Macs but please confirm whether or not the two drives are the same model from the same manufacturer.

I guess the first thing to do is check the photos where they are stored on your drive and make sure they are there and not corrupted.

Try opening some of them in the Photoshop Elements Editor.

Your catalog could have gotton corrupted, but that doesn't affect the actual photos, just the Organizer keeping track of them.

The Organizer doesn't actually physically store any of your photos, it's catalog is a database that tells the Organizer where on your drive the photos are.

Photos become missing in the Organizer whenever they are moved, renamed or deleted outside of the program. The Organizer is simply a database that keeps track of where you store them.  If you change the manner of storage, the Organizer will lose track.  I believe I have also seen problems with Macs when the OS has changed and file permissions get altered.  Hopefully, @Jeff Arola will come by this discussion and give you better information than I can provide about your Mac machine.

If you go to the file info panel and click on the location link for the missing file, does the folder where the file is supposed to be open?  And is it there?  This is the location where the Organizer catalog thinks the file should be. 

If you have moved or renamed the file outside of the Organizer, after importing it into the catalog, the file will be considered missing.  All file management (move, delete, rename) must take place in the Organizer.  Does this explain what might have happened to your files?

What version of Elements are you using? And what Mac OS? 

Where are you storing the photos that have what I assume is  the missing file question mark?

Since I know almost nothing about Macs, I'm going to ping @Jeff Arola, another community expert who will be able to help with Mac-specific issues.  (I'm guessing it is some kind of file permissions/access issue but we need more details of what you are doing that may have changed.)