I had the same issue with My Passport Ultra 1TB (HDD) by Western Digital. It has a USB-C connector but I could not find it in my files whenever I connect it to my iPhone 15 pro max. However, I removed the password of the drive by using my PC and connected it again to my iphone and suddenly the drive worked perfectly!
I am not sure why the drive did not work when it was encrypted with a password, but if your drive encrypted with a password and not working when you connect it to your iPhone then I recommend to remove the password and try again.
Hope this will help you and solve the issue!