The Local Folders location is defined for the profile. You could add an new profile and have its Local Folders defined on the USB drive. Select Help/Troubleshooting Information and click on about:profiles at the bottom of the 'Application Basics' section. Name the new profile e.g. USBdrive. Add an account and set the Local Folders location under Server Settings. Profiles can be launched from the About Profiles tab, or with the ProfileSwitcher add-on.

1. symlinks are a bad idea

2. There is one storage locations "per account" - so you can change the location of "Local Folders", but you cannot specify more than one per Thunderbird profile

What is your use case/business case for "needing" what you asking for?

The justification is that I already have a Local Folders area defined which is local to my system. I had access to another imap server location on a unix server where I kept additional mail folders for projects etc. That is now going away, so I'm moving all of those folders to a USB drive. I don't have adequate space on my internal laptop SSD to keep these mail folders so looking at keeping them on encrypted external USB drive. Hence the reason to have a secondary Local Folders area. Also this drive may also be shared out with other colleagues down the road.

I will attempt to follow the other suggestion of using another profile and ProfileSwitcher add-on.

I want a folder per year to store older mails. Where every year is stored in a separate file (like the pst files in outlook). When not used they are on a separate harddisk en when I want to find something in them, I hook up the external drive and search for the one I need. Advantage is that you don't have to backup the complete mail history over and over again and thunderbird keeps looking tidy.