I am running Boinc on a Raspberry Pi 100. I have limited space on the startup SSD, but I have lots of room on a USB SSD. I'd like to tell Boinc to use that drive instead of the startup drive. I can't find any configuration in the UI for this. Is there a config file I could edit? If so, where should I look for this file?

This s going to be a little bit messy, I'll give it a shot:

Assuming you're running some type of Linux, this is the easiest way (via a shell command window):
https://askubuntu.com/questions/117309/how-can-i-install-any-package-in-a-particular-directory-with-apt-get#:~:text=You%20can%20use%20dpkg%20instead%20of%20apt-get%20which,--instdir%3Ddestdir%20where%20destdir%20is%20your%20desired%20installation%20directory.

-Otherwise, you'll have to move the BOINC directory to your new device ("mv" command on Linux), and set a symbolic link to the new location where BOINC was originally installed on your device. Something like this:

$ cd <directory that contains the BOINC folder, wherever that is on your device>
$ mv ./<BOINC folder> <target device/directory>
$ ln -s <old BOINC folder name> <target device/directory>

The massive drawback concerning manually linking the BOINC directory to a new device is that the program or a project task may look for files in certain places, and may throw errors if the BOINC program or project tasks can't find what it needs because of the move. If you run into such a situation, you'll have to manually poke around in the BOINC directory's configuration files and see what directory settings need to be changed.

Similar (and potentially cleaner) results can also be obtained through uninstalling BOINC (if necessary), executing the mount(8) command, then re-installing BOINC. See "https://manpages.ubuntu.com/manpages/jammy/en/man8/mount.8.html".

It may also be necessary to use the "sudo" or "su" command in combination with the above commands if the account that you're using on your Raspberry Pi doesn't have the privileges to read and/or write to the USB drive and the location of the BOINC installation.