partition creation fails during something else install 18.04

I'm attempting to install Ubuntu 18.04 onto an external drive for a laptop using a USB bootable flash drive. The laptop has WIN 10 installed and I reallt want to have the setup so that if the external drive is attached, I'll boot into Linux and if not, I get Windows. When I get to the section where I define the partitions for the drive (/boot 1024 MB primary, swap area 2048 MB logical, and the rest assigned to / also logical) the swap area creation seems to fail every time. The boot record disc is selected as the same external drive. Any suggestions?

Tom