OSX Photos is Forcing Me to Optimize Storage With Plenty of Free Space In order to free up space on my macbook air's internal SSD, I moved my photo library off to an external disk (actually a Transcend JetDrive installed into the SD Card reader port). I have the photo library on the extrnal disk set as the System Photo Library in Photos Preferences, and want to re-enable iCloud Photo Library (which I turned off while I moved the library to its new location). When I try to check the box for iCloud Photo Library, I get the error message "Your full-resolution photos and videos stored in iCloud may not fit on this Mac" and the only options are to cancel or Optimize Storage. 

The library shows up as 60.59GB in Finder, and icloud.com says I have 44.99GB of Photos and Videos (which should be the exact same photos, plus or minus a few new iPhone photos taken in the past few days). The external drive is 128GB, and the Photo Library is the only thing on it.

What's going wrong? Why doesn't iCloud realize that it has all this free space to work with? How can I re-enable iCloud Photo Library without deleting photos from my macbook?
Two things to note:


the small print under "Optimize Mac Storage" states that "Originals will also be stored on this Mac if you have enough storage space", so hopefully the fact that you have plenty of space to spare will qualify as "enough storage space" and nothing will be deleted immediately (hopefully!)
even though you're not offered the option in the dialog, once you select "Optimize Storage", you can immediately switch the radio button back to "Download Originals to this Mac"

Neither of these things necessarily guarantees that the originals won't be removed and re-added, but at least you will end up with all the originals on your Mac once it has had a chance to synchronize everything to its satisfaction.