There are no conventional external drives that you can use with any Apple iDevices.
You need to transfer your iDevice data to your Mac, using both iTunes and the macOS Photos app to transfer and store photos ( you can create a Photos Library folder onto the external hard drive ) to store all of your iDevice photos to this Mac connected external hard drive.
There are a few special storage devices for mobile devices. There are portable, bulit-in WiFi hotspot hard drives for mobile devices made by Western Digital and Seagate.
http://www.wdc.com/en/products/products.aspx?id=1330
http://www.wdc.com/en/products/products.aspx?id=1660
http://www.seagate.com/consumer/stream/
There are special mobile lightning connected and built in WiFi Hot spot, USB flash drives made by SanDisk (now owned by Western Digital)
https://www.sandisk.com/home/mobile-device-storage/ixpand
https://www.sandisk.com/home/mobile-device-storage/ixpand-mini
https://www.sandisk.com/home/mobile-device-storage/connect-wireless-stick
http://www.adamelements.com/iKlips/iklips.html
This listing maybe incomplete and somewhat outdated, now. So do an online search for similar external data storage devices for iPads
Kingston Electronics makes some data storage devices for iPads/iDevices, now, too!
https://www.kingstongo.com/us/usb/datatraveler-bolt-duo-flash-drive-for-iphone
You can copy photos/images and videos/movies to these special drives, first connected to your Mac, then use the drive maker’s special iOS App to access these drives and compatible data from your iDevice.
Good Luck!