How to download photos to external drive

If the iPhone has an Apple Lightning connection port, you can use a SanDisk iXpand flash drive to use as an external hard drive. They are available in various sizes.

You can also use that iXpand flash drive as a tool to save the photos to a real external hard drive afterwards.

iXpand Flash Drive are available at Apple Store and at Apple Authorized Service Providers.

iXpand flash drive app can be downloaded from the App Store.

See -> SanDisk iXpand-flashdrive Go -> SanDisk 64 GB iXpand Flash Drive Go - Apple

Then download the SanDisk iXpand Drive app from the App Store, see -> 

https://apps.apple.com/us/app/sandisk-ixpand-drive/id1079870681

You can also transfer to a pc first, then from the pc to an external hard drive.

(PC & Mac) Transfer photos and videos from your iPhone or iPad to your Mac or PC

Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

See also -> Complete Guide to Importing Your Photos in Windows 10.