First, let try to understand together which operating system supports which file system, just to make sure we are aligned on why we are formatting the external hard drive –
Mac OS and Windows came up with different file systems, so Windows can only use the FAT and NTFS file systems, while Mac OS uses the HFS file system. Windows operating systems cannot read or write to HFS+ file systems. Mac OS can read NTFS formatted hard drives, but cannot write to them – so actually, in most cases it’s useless anyway.
So, if we want to work with the hard drive in Mac OS, we actually need to format it to HFS+. If you want to avoid formatting the drive to avoid losing data, read the part after this tutorial about a third party tool that enables full read and write access to NTFS drives on Mac OS.
To reformat the hard drive to HFS on Mac OS, follow these steps (WARNING: Please note that the process above will delete all data from the hard drive, so if you have any important data there, perform a backup before starting this process):
In Spotlight Search (click Command + Space), type Disk Utility.
Once Disk Utility is open, choose the drive to format on the left pane.
Once the relevant drive is Chosen, choose Erase from the top Menu, and make sure that the select Format is OS X Extended (Journaled) – which is actually another name for HFS+.
Then, click Erase and the formatting will start. Once the process completes, you have a drive that is formatted with the HFS+ file system.
If your goal is to have your external drive working on both Windows and Mac OS, we suggest that you won’t format the drive, but consider using Mirvago NTFS Mac Mounter. This software will enable full read and write access to your drive on both operating systems, without losing any data or having to reformat the drive.