Please try the following:
1. Disconnect both hard drives from the Pi and delete the mount points in the filesystem/mount plugin.
2. Connect one of the two hard drives, update the plugin page and save the mointpoint for the hard drive.
3. Go to the Squeezebox web interface and the server settings.
4. Enter /mnt/extdrive as your media folder and click Apply.
5. Click on Rescan media library.
6. You should find your music from the hard drive under „My Music“ after the scan.
7. Create a Samba share to the folder (/mnt/extdrive): see https://www.max2play.com/en/wiki/creating-a-sambashare/
8. Repeat with the second hard drive
Does this work for you?