I would like to be able to save my InfluxDB database file from the add-on store on a USB stick to avoid losing data if/when the sd card of the raspberry pi gets corrupted. For that we need to have a way for add-on to access mounted USB devices on the system. Can this be added?
Have you found a possible solution?
This would be such a wonderful feature!
Please look into it!
Anyone having a solution yet? 
When I used to use HassIO I used an external MariaDB database, this could be an option
This would be great! I would love to record my MotionEye and Tvheadend recordings to a USB Stick / USB HDD.
Moving to an external USB HDD (SSD) is an excellent choice (I can testify;). Please check this link for more info: HASS.IO -> transfer from SD card to SSD or USB