Best external hard drive to back up iPhone and iPad?

If you want to back up an iOS device to a physical drive, you'll need to use iTunes on your computer. You can't plug a hard drive in to your iOS device and back it up directly. You can back up your device automatically to iCloud.

How to back up your iPhone, iPad, and iPod touch - Apple Support

There are drives the will let you, to a limited extent, transfer files from the iPhone to the drive, using an app. You can't, however, perform a back up.