Unable to Install Big Sur Beta 2

I have Big Sure Dev Beta 1 installed on an external USB-C SSD drive. Mac is a brand new 2018 Mac mini (most recent Intel model). Beta 1 works fine (well, for the purpose of beta testing anyway

Big Sur Dev Beta #2 is out now. Software Update GUI Pane and CLI both see the new update, but I'm unable to downed/install it.

Here is the error I see from running 'softwareupdate -d -a' from Terminal:

'Failed to download & prepare update: Error Domain=SUMacControllerError Code=7722 "[SUMacControllerErrorPreflightPrerequisiteCheckFailed=7722] Failed to perform PreflightPrerequisite operation: com.apple.MobileSoftwareUpdate.preflight.error(Preflight):MSUPreflightErrorTargetIsNotSnapshotBooted(6) | The volume can not be used because it is not snapshot booted.'
For what it's worth, I'm getting the exact same error.

I installed Big Sur on a separate partition so as to not mess up the ability to update Catalina in the other APFS container at a future date.
Unable to Install Big Sur Beta 2
 
 
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