Hi, First, I've been a somewhat technically proficient Mac user since 1984 but this has me really stuck. I recently bought a pair of Seagate 8TB drives to use as primary and backup of project volumes. I also created separate partitions for Yosemite, El Cap, and my current Sierra system. I have cloned my main internal boot volume onto both Seagates, yet when I tested to see if they were bootable by restarting and holding down the OPT key, neither of these showed up as possible boot volumes. Only the"Macintosh HD" showed up. I used Carbon Copy Cloner, which has worked fine in the past. My old 2TB backup drive still shows up when I connect it. Any helpful tips? I'm conversant with Terminal if that's the way to go. Thanks!