How to get access to USB Disk from the Files app

USB Disk can appear in the "Location" section of the Files app, but you need to enable it in the Files app. To do so:

Once USB Disk is visible in the Files app, you will not only be able to access it from that app, but also from any app that uses the system file browser. For example, in the Mail app you can attach a file from USB Disk to an email. In Safari, when choosing a file to upload, you will be able to choose a file from USB Disk.