I successfully installed and ran High Sierra 10.13 dev beta 5 on a Thunderbolt 3 external SSD drive, from which I had booted my Mac when the external drive was a dev beta 4 startup disk. Then I rebooted into the Sierra 10.12 internal drive. All this was on my MacBook Pro (Late 2016) with Touch Bar.
Then I discovered this astonishing entry in the dev beta 5 Release Notes: "Volumes with High Sierra beta 5 installed can't be mounted or selected as a Startup Disk on systems running earlier versions of macOS. (33060569)"
Sure enough, now I can't find any way to boot from the external drive to continue working on my High Sierra application development.
While booted from the 10.12 internal drive, I can use the Startup Disk pane of System Preferences to select the external drive, which is mounted and readable on the desktop and visible in the Startup Disk pane. But when I click the Restart button, I get an error sheet saying "You can't change the startup disk to the selected disk. The bless tool was unable to set the current boot disk."
If I choose Restart from the Apple menu and then hold down the Option key, I eventually do see both the internal drive and the external drive, and I can use the arrow keys and the Return key to select the external drive and restart. But, after a lot of flashing lights on the external drive, the Mac restarts into the internal drive, instead.
If I shut down and then start back up from the power key while holding down the option key, I eventually see the internal drive and the external drive just like on a restart, but again the Mac eventually boots into the internal drive, instead.
If I hold down the C key after restarting to force a boot from the external drive, the Mac simply never boots, even if I let it keep trying for several minutes. Once I let up the C key, the Mac quickly boots into the internal drive.
If I hold down the D key after restarting, the Mac goes into diagnostics mode and takes 3 minutes to check everything. It then reports "No issues found."
Am I stuck? No more High Sierra development work for the next two weeks, while Apple works on dev beta 6? Or is there some other way I can boot into dev beta 5 on this external drive?