Read this article on working with referenced media. These are not synced via iCloud Photos and won’t be copied with a non-synced Photos library, either.

One complicating factor for both kinds of copying has to do with referenced media: these are included in Photos as links to images or videos stored elsewhere on the Mac using Photos. Turn on your camera and connect it to your computer using a USB cable. Since the images will be identical in iCloud and in the Photos library, they should all match instead of requiring massive throughput consumption and bandwidth use. Transferring video from a camera via USB PC & Mac 1.

However, my analysis while testing is that only a comparison is performed instead of a full download or upload of images. If you copy the Photos library in that scenario and then enable iCloud Photos on the new Mac, Photos will still appear to perform a synchronization operation. It can be a better choice if you have a Photos library with tens of gigabytes of images or more paired with low-speed broadband (in the tens of megabits per second) or a monthly usage cap beyond which you pay overage fees or have your service throttled to a lower speed. That second option plus copying works because you have a full copy of all images present in your iCloud storage. But you can choose instead to copy your Photos library as if you weren’t using iCloud Photos at all. With the device powered on, insert the Lightning end of the Lightning-to-USB cable into the port at the base of the device.

If you set Photos to keep full-resolution downloads of all images on your current Mac, you can also enable Photos for iCloud syncing as in the previous point.

This can take a while and consume a lot of internet bandwidth. Photos automatically performs all the downloads necessary.

If you have optimized media enabled on your current Mac, you can simply use the same iCloud account on your new Mac and set Photos up for either optimized or full-resolution mode.

When you want to copy or move your Photos library from one Mac to another with iCloud Photos enabled, that presents you with two choices:

How to transfer photos from imac to usb