The headline might be a little misleading, but trying to install the Apple OS Sierra Beta led to a series of events that seems to have rendered my MBP inoperable for the moment.
As I did with the previous release, El Capitan, I signed up for the public beta. With El Capitan I did have access to the developer pre-release, but when I did that in the past, it resulted in a ton of frequent pre-release updates. None of those caused problems, just annoyances as there were occasionally several pre-release updates for the same thing (RAW camera...) before a real release would be made.
I got my notice that I could install the Sierra Beta update on Friday. So I did what any eager nerd with part of a day off would do...I started to install it.
I've been bitten in the past, so I carefully considered that I'd just had a Time Machine back-up a couple days before, and had done nothing of value on the machine since. I ran a quick Disk Utility check, and all was well. I downloaded the beta, started it over when my (not at home) WiFi died, did it a third time, and ultimately began the update.
The update checked itself, found it to be in order, unpacked it self, checked that was in order, and then ran a disk scan on my selected hard drive. It then found an error from which it couldn't recover. I suspect that the previous successful disk scan means that there was some quirky but revealed with the expansion of another GB of data.
I restarted in recovery mode, single-user mode, recovery mode again... I ran the Disk Utility, fsck, more Disk Utility. I rebuilt the catalog (which it was complaining about) and other things. I even tried to find how to stop the updater from running--it seemed all it had done was expand stuff and set up a boot-to-update process, which seems like it should be reversable.
Finally I thought, "screw it, erase the volume, do a reinstall of El Capitan from the recovery partition, and restore from Time Machine when I get home." Erasing the partition resulted in an "untitled is not journaled and cannot be resized" warning when attempting to create a new partition. The new partition didn't exist, couldn't be erased, and re-attempting to create resulted in the same warning.
I punted and returned to the restore menu and tried the reinstall OS option again, hoping the installer would perhaps have magic partition luck. It did not. It did warn me that my hard drive was now giving SMART errors and that it needed to be replaced.
I figure that's unrecoverable, so I'll be making a trip to the hardware store on the way home. 17-inch Mac Book Pros are getting hard to find.