Photos: how to add a copy of a picture from one album to another, so it resides in both.

What kind of album is "Album X"? Is it perhaps a Shared Album? Shared albums are occasionally somewhat unresponsive and will not let us drag a bunch of of photos together to a standard album, if there is a photo among the selected items, that has already been imported from the Shared album.

You can easily join your albums by creating a smart album (File > New Smart Album) with the rules:

Match any:

• Album is Z
• Album is X
• ... etc.

Is this a smart album you've created? If so what are the criteria of the album?

If the normal reboot doesn't help give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Just drag the photo from wherever it currently resides into Album C. It does not copy the file but puts a pointer to the original file. It's like playlists in iTunes. You can have an audio track in many playlists but only once in iTunes. It's the same with photos and albums.