There are a couple of reasons why your iPhone Duo won't see a connected external drive, but more importantly, there's a solution. So you have an external drive that plugs into the USB-C port of your old iPhone perfectly fine. But when you connect it to your iPhone Duo, nothing happens.
Not to build up suspense here, there are two possible solutions. One is that if you have the iPhone Duo in a case, take it out and try again. The other is to change the USB-C cable. You will typically have had an extension cable when you bought the drive. The odds are that this will do the job, and so would other USB-C cables.
In AppleInsider testing, the problem came up with a Lexar drive that had a USB-C connector built in. The drive came with an extra cable intended for camera connections, and that worked. The problem is definitely due to the length of the USB-C plug and the depth of the USB-C socket.
It's less convenient carrying an extra cable as well as an external drive. But the fact that you can do it means you could save money and buy an iPhone Duo with less storage than you're used to.
Source: AppleInsider · Summarized by HeadlinesBriefing