How do I access an attached thumb drive?

I've attached an external drive to my phone using a USB-OTG cable. I can open it in File Manager (v2.0.0), but when I go to /storage/usbdisk none of my files show up, and if I try to create a file it complains about needing elevated permissions. I've read online that I need to enable root access mode in File Manager's general settings, but there is no such setting on my device.