I have iCloud 2TB and love it. (EDITs for clarification: Everything is backed up to iCloud - several macs, iDevices and a large shared Photos library.) However I don't trust iCloud 100%. I want a local backup of Photos Library. I plan to get a second HDD, a 4TB 2.5" model, connect it to my mini and partition it like this: 2TB for a local copy of iPhotos Library from all my family's macs & iDevices - using the iCloud Photos "download originals to this Mac" option - this will be in APFS. 1TB for Time Machine for my family's macs. This will have to be in HFS+ until Apple revamps or drops TM. 1TB for Windows files - Steam games / movies - this will be in NTFS. Am I right that all I need to do is format and partition the HDD in Disk Utility in GUID, and format the NTFS partition in Windows, and all should be fine? I'll probably want to tweak the partitions in a year or so, should I have the NTFS (or HFS+) partition first or last in order to be able to non-destructively repartition the drive? EDIT : Just to be clear, this is a second-level backup as insurance against iCloud going down, and also a place to put large downloaded files, nothing irreplaceable.