There are 2 partitions on the LaCie external HD. I have cloned to both partitions in the past using SuperDuper and have booted into both of them at one point or another. I use 1 partition as a permanent clone backup which I do every week to 10 days. The other partition where I was trying to install Sierra has been booted from in the past although it has been empty for at least the past 6 months. I erased it using Disk Utility and set it up for Mac prior to trying to install Sierra.
