It seems this behavior is gone in beta 3 of Big Sur for the other macOS partitions. Now it only unmounts root. Start up
disk utility to see this. Disk utility can now mount the system R/O volume unlike in beta 1 + 2
jteloh
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I had this problem after installing beta 1 and I solved it by deleting ~/Library/mail*. There should be three of them.
My first attempt was like everyone else. A spare partition of 256GB on thunderbolt drive. So I had good backups and decided to install on Internal m1 Mac mini disk. This erased the installer so in terminal I:
softwareupdate --fetch-full-installer
which downloaded the installer. The first time I ran it, it errored out. So I ran it again and it installed and boot the new part.
This makes me wonder about the deterministic nature of this. So if you run Monterey, you can get an external disk running it.