Many external hard drives come pre-formatted as FAT 32. This is a native Windows file format that can be read by macOS, but is not ideal for use with a large files on a Mac

1. In the Finder, choose Go > Utilities
2. Launch Disk Utility.
3. Click the icon for your external hard drive in the sidebar on the left.
4. Click the Erase tab along the top of the window.
5. From the Format menu, choose Mac OS Extended (Journaled).
6. Enter a name for the external hard drive in the Name field.
7. Click the Erase button.