Depending on if you are using your external hard drive with Windows, Mac, or both, the ideal file format will vary. If you're not sure what format you should choose, you can consult Apple's ...

Whether you're trying to repurpose an old hard drive ... your USB drive should now be formatted as FAT32 and ready for use.

Buying an external hard drive or solid-state drive for your Mac is much like buying one for your Windows PC ... drives can be had for less than 10 cents per gigabyte—and often much less.