I have a late 2102 Mac mini with a 2.12 TB Fusion drive, and a 2015 MBP with a 256 GB SSD. My Photos Library, which was on the mini, is 110 GB. I have Apple's 200 GB iCloud storage plan, and my photos are backed up there. I've decided to sell my mini, so I copied my Photos Library to an external drive, signed out of iCloud on my mini, attached the external drive to my MBP, and designated my 110 GB Photo Library as my System Photo Library. I got a message saying I need to sync it to iCloud, so I went to my iCloud Preferences and selected the "Automatically upload your entire library to iCloud ..." option. It tells me that I don't have enough free storage and that I need to upgrade to the 2 TB plan. Apparently, iCloud thinks that I'm adding 110 GB worth of photos to the 110 GB that's in iCloud, when in fact it's the same library. I'm running High Sierra and the latest version of Photos. I'm guessing I need to unsync my iPad and iPhone from iCloud, delete my entire Photos library from iCloud, sync iCloud with my System Photo Library on my MBP's external drive (thereby uploading all my photos again), and re-sync my iPad and iPhone. Is that correct, or am I missing something? I've moved my System Photo Library before, and this never happened.