Today (4/6/18) I performed the latest security update for OS10.12.6 Security Update 2018-00 bundled with Safari and iTunes updates. After restarting, it was apparent that my 2012 Mac Mini that normally boots from an external USB 3.0 SSD was instead booting from an old OS living on a partition on a Firewire 800 external drive. The OS was circa 10.6. . . When I checked my startup drive options while in that state, the external USB 3.0 SSD was not available or even listed as an option. So I restarted in Safe Mode and the SSD reappeared and was the default startup drive. Just to be sure, I reselected it and then ejected the external drive with the various bootable volumes and powered it down. After I restarted again, I powered up the FW800 external drive because it contains files that my apps access. Things appear to be working but I have not tried restarting with the external FW800 drive powered-up. BTW, the OS Sierra Security update that I applied is not showing up on my App Store list of updates done within the past 30 days. However, both iTunes 12.7.4 and Safari 11.1 that were bundled with it show up, with an install date of March 31, 2018 instead of April 6, 2018. I'd appreciate insights/feedback about what, if anything, I should do next. . . At some point later today I'll give the restart a try, since apparently I can recover from it if the same thing happens again.