Mac mini Photos stuck syncing after SSD move

This should be the default but make sure that Finder > Get Info of the external drive it has "Ignore ownership on this volume" at the bottom Sharing & Permissions. Also make sure the drive is formatted as case-insensitive.

Make sure the drive is connected directly to the Mac (no hubs) and try different ports and cables. Is the drive self-powered or does it have an external power supply which is more robust? Check the drive with Disk Utility. Does a detour to safe mode help? Do not use any 3rd party "cleaners", VPN or virus apps that often produce obscure errors and corruption.

Wombat26 wrote:

All of this is as I have it.

I do have CleanMyMac X installed and run that every so often.

Could this have caused an issue?

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu).  

Additionally, a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.

Follow leonie and Matti's recommendations about removing CleanMyMac and its supporting files. You can use the shareware app Find Any File to search for any files with a particular file name.  For CMM software you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macpaw

Any files that are found can be Option dragged from the search results window to the Desktop to get a copy of the file.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Make sure you set the Photos library on the new SSD as your System Photo Library in it's General Settings pane:

A key question in all of this: what format are those drives? They need to be formatted Mac OS X Extended, Journaled or apfs.

Wombat26 wrote:

I do have CleanMyMac X installed and run that every so often.

Could this have caused an issue?

"Cleaners" like that as well as 3rd party virus apps are infamous for producing obscure errors and system corruption. Sometimes backup, erase, clean install and carefully restoring only user data files is needed to fix the damage they have done.

Just be patient. Don't change any iCloud Settings for Photos. That could cause a new sync from scratch all over again.

But it would help restart all Photos background processes occasionally by signing out of your user account and signing in again.

added: when I tested the trial version CleanMyMac it messed around with the originals in the Photos Library and caused many incorrect links, that even the Library repair tool in Photos could not fix. I hope you still have a backup copy of your library from the time before you started using Clean My Mac.

How long ago has it been that you moved the library to a new drive? Your library is huge, so it may take a very long time to get it fully synced again.

It would have been more economic to start over with a new, empty Photos Library and not sync the copy of the old library with iCloud. Now iCloud Photos has to upload all photos again, that are already in iCloud to compare them with iCloud. By starting over with an empty, new library you would have avoided the upload and just downloaded the library from iCloud as it is now. This way it would have been easier to see the progress.

Do you see these videos at iCloud.com? Do you see them on your phone? That is, is the problem between the phone and iCloud, or is it between iCloud and the Mac?

Videos can be a problem. The codecs change without warning, and .mov doesn't always mean the same thing. And videos seem to be more sensitive to corruption-- for video, it's still the Wild Wild West out there, and there's no law West of the Pecos. (or East, for that matter…)

Can you run the videos at iCloud.com? (I swear-- iCloud.com seems flaky on Safari, but it works in FireFox for me-- don't know why…) A single corrupt file can interrupt all downloading, so maybe if you find it, you can start up the process again.