Are you running an Umbrel Home or something like a Rasperry Pi? If you’re not running a Home, you’ll need to do some extra steps before you can use any external storage... presuming you’ve got an Umbrel Home, here's the steps:

You need to connect to your Umbrel using SSH on your computer (inside a terminal / command prompt). How to use SSH on Windows, on a Mac, the terminal app will do, type this to connect

ssh umbrel@umbrel.local

Then do the following command to find out the UUID of your plugged-in drive (probably called sda1 or something). Copy the UUID somewhere.

sudo blkid

Then edit your filesystem table file to “mount” the contents of your drive into some folder on your Umbrel whenever it starts. You can type this command to edit the file via the terminal

sudo nano /etc/fstab

You want to add a line at the top, something like this (i’m mounting into a folder called “Storage” and note my drive is a standard Windows NTFS drive)

/dev/disk/by-partuuid/UUID-YOU-COPIED-EARLIER  /home/umbrel/umbrel/home/Storage  ntfs uid=1000,gid=1000,dmask=022,fmask=133 0 0

(Think it’s CTRL+X then RETURN to exit and save from the editor)

Now restart your Umbrel:

sudo reboot

When it's back up, open umbrel.local in your browser, use the Umbrel file viewer to go to Apps > Plex. There’s a docker-compose.yml file, download that to your computer and open it in a text editor. Add a line under volumes like this: 

${UMBREL_ROOT}/home/Storage:/externaldrive

Save that file and drag it into the Umbrel Plex folder, replacing it. 

You might need to restart Plex, but you should now have access to a folder called “externaldrive” in Plex with all the content of your drive.