Destination volume on external drive with no drive letter

I want the destination of the backup to be a volume of a SCSI-USB external drive, that has no drive letter. There is another volume on that same drive, that has a drive letter assigned, and that volume shows up in File Explorer on Windows 10. However when I try to configure the destination of the backup, there's no way to specify the volume that doesn't have a letter. If I click "External drives", it says "No external devices found". Is there a way to do this?

Give it a drive letter?