iPhoto Missing Original Files preventing iCloud upload

First, do you really mean the discontinued app iPhoto, or are you using the current app Photos?

If Photos, do you intend to use a Referenced Library? Normally, in Settings (or Preferences) the choice "Copy items..." is checked, and all the pictures are kept in the the Photos Library where Photos keeps them safe. (The pictures that were copied remain where they were.) This is a Managed Library--the Photos app takes care of it.

If this box isn't checked, then it's called a Referenced Library, and Photos tries to use the pictures where they were-- but if you ever move them, then Photos can't find them. That seems to be what's going on. Referenced Libraries are notoriously unstable.

To get out of the circle of doom, the first things to try are these:

• Restart the computer (of course)
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.
• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work is also diagnostic.

iCloud Photos needs to be enabled in two places, in the "System Preferences > Apple ID > iCloud > Photos" and in "Photos > Preferences > iCloud". Have you checked both places?

You may also want to check your iCloud account page to see, if there are any problems with your AppleID, perhaps you need to update your payment method or similar.

And you may want to look again at the status bar in Photos, below the "All Photos" view: Check the status of your library in Photos for macOS - Apple Support

Does this show, if Photos is uploading?

I am not sure, if your system version offers to pause the syncing temporarily.

When you look at the status bar below the "Library " view in Photos, is there a the word "Pause" in blue? Then click "pause" to stop the syncing temporarily.

The handling of referenced files has changed between the versions of macOS.

iCloud Photos has never been able to upload referenced files to iCloud, intentionally. But the early versions of Photos just skipped the referenced images when syncing with iCloud and just uploaded the managed files. It has been a neat method to sync only a partial library to iCloud. The upgrade to macOS 10.15 Catalina has broken this workaround. Now the iCloud syncing may not be possible at all, if there is a single referenced image in our Photos Library.

Deleting the referenced files from your Photos Library should have fixed the problem. Did you remember to empty the Recently Deleted album? The referenced files must not be in Recently Deleted either.

I think, you will have to be patient do not interrupt the upload.

my library took ten days to upload completely and had less videos than your library.

While you are waiting, try to play your videos and check, if Photos can open them properly. Some older videos may need converting to a different codec, as Apple has removed the support for some older codecs from the more recent system versions.

Cheech wrote:

is there a way of switching iCloud uploads on and off as it might be best to upload over night?

On my Mac I am editing pictures on an alternate library that is not synced with iCloud, and I sometimes use an editor that needs me to set this alternate library as my System Library. So I may, sometimes several times a day, turn off iCloud in my synced library's settings. It stops updating, but starts back up when I turn iCloud back on for that library. So far that has not caused any problems. It may help that my synced library is relatively small, with only favorites and recents.

Anyway, if you don't have a "pause" button, then you can try just disconnecting iCloud.

Cheech wrote:

..

But it seems lots of Photos and Videos are dropping back into my iPhone and iPad. I'm not sure why. Is the something to do with enabling iCloud (seems odd). This will start to use up storage on these devices. If it somehow linked to what i'm uploading to iCloud from the iMac (15k Photos and 1.5k videos) they will fall over?

If you enable iCloud Photos on more than one device, iCloud will sync all libraries and keep the photos libraries on ll synced devices identical. That is the main purpose of iCloud Photos - to ensure that the libraries on all devices are perfectly in sync and you will find exactly the same photos and videos on all your devices.

Sowe should keep only those photos and videos in our iCloud Photos Library that we want to be on all our devices and keep the iCloud Photos Library small enough to fit onto the device with the least storage.