I don't know about the iPhone or iPads but I connect an external 1TB Seagate hard drive to my Samsung S7. I was surprised the S7 had enough power on its USB to power it. Still a flash drive would be better.Generally speaking, you can't run a full HDD off an iPad or iPhone. A small flash drive, such as _Pete_ suggested in his first link, but not a full HDD.
Why not network this. Either by buying a NAS hard drive and all devices connect to it via the network, or by connecting the drive to the computer and sharing it over the network to the phone/tablet.
I don't know about the iPhone or iPads but I connect an external 1TB Seagate hard drive to my Samsung S7. I was surprised the S7 had enough power on its USB to power it. Still a flash drive would be better.