I'm trying to clone a single bootable partition on my Ubuntu Linux 16.04 system to a new SSD in an external USB container. I want the new SSD drive to be bootable. Clonezilla definitely sees the destination drive, the external USB drive with that has the ID "sdd", when it loads up. It also sees "sdd" as a potential destination if I enter disk-to-disk clone mode. However, it does not see "sdd" when I go into partiton-to-partition mode. It only lists the internal partitions (sdb1/sdb2/etc.) when offering me the list of destinations t choose from. It absolutely does not offer "sdd" as a potential destination for the partition-to-partition cloning operation.
I can't use disk-to-disk clone because the source disk is a dual boot disk with Windows and Linux on it, and their combined size is larger than the destination disk. However, the Linux partition I am trying to on the destination disk is much smaller than the destination drive "sdd". How can I get Clonezilla to see "sdd", the external USB drive, as a potential destination and offer it to me as the destination for the cloning operation?