I presume the OP has a purpose for wanting multiple bootable partitions on a disk and this is certainly possible with a GPT partition scheme and HFS+ partitions.
The OP mentioned 8TB Seagate drives but didn't mention if it was external or internal. One thing I have noticed is that my 2 docking stations, which date to early 2013, can have issues with booting an OS, depending on which Mac I use and whether or not it's connected to a hub. (Even with these problems, I see them as boot options when starting up.) I have no problems with recent disk enclosures booting an OS (or having multiple bootable OS's on them). The USB-SATA circuitry on my Seagate external drives have all worked without a problem and having multiple OS's on a drive shouldn't be a problem but I haven't tested them for this purpose.
Here are some clarifications and further details:
I have a late 2012 Mac Mini. I have in the past run 10.10 Yosemite, 10.11 El Cap, and currently 10.12 Sierra. The internal drive is a fusion comprising a 240gb Crucial SSD and a 500gb HDD, making 740gb. It is not partitioned.
Since I'm doing a lot of photography and videos, plus a lot of other images and mixed media ebooks, as well as having examined all the hundreds of apps in my /Applications folder and have pared the number considerably, putting the less-used apps on one of the Seagates. I decided to go for a couple of Seagate USB 3 8tb drives from an offer on Amazon. These are the only external drives on my system. I also intend to use 3tb as a Time Machine partition.
I wanted to have specifically a bootable Yosemite partition because of a past file system problem where I couldn't delete a few files in El Cap because the file names somehow contained ~ characters and no amount of GUI or Terminal commands could remove them. In another forum someone suggested using Yosemite. I created a bootable Yosemite partition on an older external drive and was able to delete the offending files, which is why I'd like to have 10.10 handy.
So now at the present, I've tried to boot from that drive, and failing that, I tried to create a boot partition on my new Seagate, and that failed as well. Then it came to pass that even though I had used Carbon Cooy Cloner to clone my internal fusion drive to a backup partition on the Seagate, and even though both the 10.10 and my Sierra backup showed up in the Startup Disk pane in System Preferences, neither of them show up when I reboot and hold down the option key; only my internal regular boot volume shows up.
Now over the past couple of days I've restarted my Mac while holding down the option key with no change after doing the follwing: reinstalled Sierra by holding down the cmd-r combo, erased the entire boot volume and reinstalled Sierra from a USB stick and before copying my user files, and erased the primary boot volume and did a Time Machine complete recovery. No amount of reinstallation of Sierra onto my primary boot drive alters the situation.
Now here's the strange part - I took my small external drive with the El Cap installer to a friend's today and it showed up when he rebooted holding down the option key! Clearly there's something haywire with my system in that it doesn't recognize perfectly legit boot volumes, and this worries me. It worries me that I can't even boot from a CCC clone backup drive. What if this issue is the tip of an iceberg?
One final note: I've decided to forget having 10.10 and 10.11 boot partitions on the Seagates. I can use the older 1tb portable drive to set up boot partitions. At this point I just want to resolve the greater issue so my backups will boot if necessary.
I hope this helps to refine the description of my problem.
