Attempting to update macOS Catalina 10.15.7 to Monterey on 16" Macbook Pro (late 2019) via Software Update fails because "disk2s2 is not convertible to APFS: You may not install to this volume because it cannot be converted to Apple File System (APFS)."
Installer Logs show:
Jan 20 11:06:45 petrus-mbp16 osinstallersetupd[99866]: ensurePrebootVolumeForDisk: EnsureRecoveryBooter: APFS Recovery Volume recoveryVolumePreExisted=1
Jan 20 11:06:46 petrus-mbp16 InstallAssistant[99862]: OSISClient: Interrupted!
Jan 20 11:06:49 petrus-mbp16 osinstallersetupd[10341]: osinstallersetupd registered client
Jan 20 11:06:49 petrus-mbp16 osinstallersetupd[10341]: disk2s2 is not convertible to APFS: You may not install to this volume because it cannot be converted to Apple File System (APFS).
Disk Utility shows all my drives are already APFS volumes. Hmm, disk2s2
? That's the "Shared Support" drive that the updater downloaded...
Attempting to convert this drive to APFS manually fails with "internal error".
How do I update my macOS given that the official update fails?