A:Answer Depends what format you use.
If you try to do TimeMachine or use it for Photos in the Mac, then macOS will format the disk as HFS+. Windows won’t natively need able to read it in this format. If you use Windows and try to use it for File History, then Windows 10 is going to want it in NTFS. If you format in NTFS, then macOS cannot read the drive natively.
There are tools that let macOS read NTFS and Windows read HFS+ but both are reverse engineered hacks since neither company licenses their file systems.
If you use the FAT32 or ExFAT formats, you can seamlessly share across macOS and Windows.
