I can't connect my flash drive to my nexus 5x. I have the micro USB version and I used the nexus microusb to USB type C adapter. Does it not work because I am using an adapter and not an otg cable?
Yes I believe that is your issue. I had the same problem with my 5X. Two different micro USB to USB C adapters didn't work but a USB C on OTG cable does work. I found one on Amazon.
I use that type of device to move my photos from my phone to my desktop for editing. When I got my first one, using a previous version of Android, I didn't get it to work and gave up. I'm fairly new at this. Now, with Android 7, I use two that work fine. It took me a while to figure out how to make it work but it's fine now.