I had a whiteknuckle stalled update experience while restarting for the new point update (10.13.4) last night. Ultimately everything ended well, but I got hung up on the Startup Volume chooser a couple of times due to a quirk of my 8-year-old MBP 17’s optical bay SSD, and was curious about the second volume (named MacOS Update I believe?) that appeared alongside my startup disk.
Does this live in a hidden APFS volume? Or can the Mac interpret disk images without the OS being booted? What are its contents?
Anyhow, mainly I wanna know what part of my install would be messed up if anything DID go awry, as I had a number of very strange experiences such as a long tone, safe mode boot, and the Mac booting up following the alleged update still running the previous version, yikes. Thanks!