Your Mac has 128GB of storage and you have 600GB of photos. What to do?
iCloud Photos lets you keep full-resolution versions of your images and videos in iCloud storage, while letting you choose to store just optimized versions—thumbnails and low-res video previews—on your iPhone, iPad, or Mac. That’s great, especially when you have more media than storage. When you need the full image or video, you can double-click it within Photos to retrieve it for local use.
The conundrum can come when you want to load a massive amount of media data into Photos on a Mac set to Optimize Mac Storage (in Photos > Preferences > iCloud) all at once instead of adding to it over time. The trick is to stagger your import. Let’s say you have—as one Macworld reader did—600GB of media data and a 128GB disk drive in your Mac.
After setting Photos to optimized storage, import batches of data and wait for them to upload. You can follow upload progress at the bottom of the Photos window in the Photos view. You may have to bump the scroll a little to hit the very bottom where the library size is shown and the iCloud upload status displays.
Read more at MacWorld.com