Ipad pro to external hard drive

I have an external hard drive I used for my pc laptop (which has now died). I can plug the drive into my new ipad pro (no extra power source needed) with a usb-c to usb adapter. Then I can see all my old files on the ext hd and can save any photos through the Files app to my ipad photos. But, when I select photos from ipad photos and share, save to files it lets me open all folders in the ext hd but the save button is greyed out.

The same occurs if I use the Files app. So I can only copy files and photos one way, from ext hd to ipad but not the other way.

What I really want to do is use the ext hd as a photo backup. I can’t tell how it is formatted because the laptop died. 

Any ideas??