Also, you can find a few external lightning USB drives such as the Sandisk iXpand Flash Drive and consider this as a not cheap but great solution to save your MP3, MP4, M4V, PDF, EPUB, or JWPUB files to your iPhone or iPad.
Well, the lightning ports on those iXpand drive does work with the Files app on iOS 13 and iPadOS 13 but not directly but via an app that made by Sandisk called SanDisk iXpand™ Drive. Without that app, your iXpand drive will just fall short to be accessed by the operating system. The reason is because Sandisk iXpand Flash Drive, or Transcend JetDrive Go, or similar devices are not MSC and just use iAP2 to communicate with iOS applications.2
The ideal is Sandisk, or Transcend and others will keep on updating their app to make the lightning port of their USB drives work with the latest iOS/iPadOS. If not, you have to use its other end, which is a USB Type-A and then hook it to the Lightning to USB Camera 3 Adapter. This is terrible since you paid a high price for the lightning compatible function and have to pay again for an adaptor just because it does not work.
Or maybe, just don’t invest into any lightning USB drives, but purchase the Lightning to USB Camera 3 Adapter and plug any USB Type-A drive that you had already.3
If you have an iPad with a USB Type-C port. Congratulation, it will work native with any USB Type-C drive and/or USB Type-A drive + any USB Type-A to Type-C Adaptor.
Using the Lightning to USB Camera Adapter, Lightning to USB 3 Camera Adapter, USB-C to SD Card Camera Reader, or Lightning to SD Card Camera Reader ↩
Although iOS 8 was iAP2 ready, the latest Sandisk iXpand Drive app requires iOS 10.0.2 or later and Transcend JetDrive Go app requires iOS 11 or later ↩
I recommend this instead of the Lightning to USB Camera Adapter for better speed and having its own power source ↩