if you feel comfortable with the linux command line, you should be able to move the app directory to your second hard drive (make sure it automounts) and then symlink the umbrel app dir on your primary hard drive to the app directory on the second drive. Works for me.
Now be prepared for when you upgrade because it will not respect that and you will need to do it again. It would be much easier if they just let you configure manually which directory to install things to