What is the problem?
Octoprint only shows files on the printer's SD-Card or from inside the internal upload-folder. The filelist of these files don't get refreshed automatically.
What did you already try to solve it?
I installed usbmount to automatically recognize and mount hotplugged usb-sticks and sd-card-readers. I had to modify usbmount's config-file to mount new devices to subfolders of octopi's upload-folder.
This way the octopi-file-manager can show the files on usb-stick, even though it doesn't support additional storage-devices natively. Softlinking to /media/usb0, usb1,... doesn't work. Octoprint can't access the files then.
So I created two folders: USB1 and USB2 in /home/pi/.octopi/uploads/ and configured usbmount to use them as mountpoints for sticks and card-readers.
Only problem now: octopi doesn't recognize a removal of a stick. It still shows the files in the mount folder after unplugging stick and/or card-reader.
So this bug-report is rather a feature-request: Please add an option to auto-refresh the file-list on hotplug/removal of USB-sticks.
A good reason for this are the poor capabilities of accessing the printer's SD-card: Slow speed, short-filenames only. A card-reader connected to raspberry and octopi was the better alternative.
Especially when using a touchscreen with Octoscreen or OctoDash, SD-card- and stick-support was preferable.
Have you tried running in safe mode?
Did running in safe mode solve the problem?
Additional information about your setup
OctoPi 0.18.0 with Octoprint 1.6.1 and OctoScreen 2.7.4