No USB devices available in VirtualBox

Please add your user name to the vboxusers group with this command:

sudo adduser $USER vboxusers

After that you must logout and login. (For Ubuntu 20.04, a reboot is required)

Please check this for more details:

https://help.ubuntu.com/community/VirtualBox/USB

If you don't have the adduser command, you can do this instead:

sudo usermod -aG vboxusers $USER

Logout and login again in order to reload user's group info and usb device will now show up in the list.

First of all, @csorig's answer is right. You need to be in the vboxusers group. That's the basic.

But if it still doesn't work for any reason... it's not documented anywhere, but I found that USB host device sharing does not work if the system has run out of inotify resources.

You can try running tail -f /var/log/syslog or something like that. If it shows up a message like:

tail: inotify cannot be used, reverting to polling: Too many open files

then you need to increase your inotify watch limit or disable software that is consuming them. In my case it was a continuous backup software running in background.

The basic method to increase this limit is:

echo fs.inotify.max_user_watches=524288 | sudo tee -a /etc/sysctl.conf && sudo sysctl -p