Can't select drive letter on external hard drive

I partitioned a 3TB hard drive to have two partitions and I would like to flash recalbox to only one of the partitions. However, when I go to select my hard drive it won’t let me select only one partition. They both have drive letters assigned to them so how do I select just one partition? I’m using Etcher 1.4.4.

Flashing disk images to partitions isn’t supported by Etcher at this time, and given that disk images are full-disk images, and contain their own partition tables, bootloader code etc. this would involve quite a bit of manual fiddeling to pull off, and might even be rather hard if the filesystem in the image isn’t resizable or isn’t implemented by your host OS.