Unable to sync my 100gb iCloud external harddisk because my Macbook harddisk to too small.

Have you set the library on the external disk as the System Photo Library? That's a button in Preferences > General while that library is open. Look on the line between "Library Location:" and "Memories:".

If the button "Use as System Photo Library" has the same appearance as "Show in Finder", click "Use as System Photo Library" to make this library the System Photo Library, then switch to the iCloud tab and try enabling iCloud Photos. Expect a re-sync of your library with the cloud.

If the button "Use as System Photo Library" is dark-ish (hard to read), then this library is already the System Photo Library, and you should be able to use it with iCloud Photos.

the advice by markwmsn is not quite helpful. I upgraded to Catalina. I have more than 600Gb of photos stored in a Photos Library on an external HD I set and used as System Library for years. After the recent Upgrade I can not connect this Library with iCloud by checking the box in Photo preferences. Dialog offers „Optimise Storage“ which - thanks but no thanks- I never asked for.

Fact of the matter is my System Library and my iOS devices are not connected any more.

Yours is to quite the same situation, glenrothes, though it is related. I would not expect the same explanation to suffice.

As you said, your local library has gotten disconnected from your iCloud Photos library for some reason. I would not expect an upgrade to Catalina to cause such a disconnection necessarily, even with a library on an external disk. (My library is also on an external disk, and did not suffer such a fate when upgraded from Mojave to Catalina or among Catalina versions.)

Once the libraries become disconnected, iCloud Photos will attempt to synchronize them by merging them. Before it can do that, it has to make sure you have enough space on both ends. In the worst case, Photos might need to download the entire iCloud Photos library content to your System Photo Library. Rather than get partway through, Photos throws up the error message you saw.

How much free space do you have on Raid4TBBay1? The message suggests it is not enough to add your entire iCloud Photos library content, but it should not need to do so if these are indeed copies of the same library.

If you have plenty of free space there (even if not 600GB), you could enable Optimize Mac Storage for the duration of the synchronization, then turn it off. Experience shows that Photos won't optimize any photos already on your Mac until free space gets low. (People complain here about not seeing immediate optimization.)

Although experience also shows that newly downloaded items will be optimized at download, there should not be many new items if your libraries have been disconnected only a short time.

Yes , had the same problem and what you say is the solution. Thank you !