For some tests, I need to deploy a precise setup: MacOS 10.15.4 with Xcode 12.3. My MacMini 2018/2020 is running 10.15.5 with VirtualBox 6.1.12. I have a running VM with 10.15.0 which I snapshot after installation. My plan was to then do a manual upgrade to 10.15.4 thanks to the corresponding combo update, as I do not want to jump to the latest Catalina version.

The process of the manual update is to mount the dmg file, which then shows the pkg file that starts the upgrade process when run, and asks to reboot at some point to finalize everything. The reboot doesn't take me to the upgrade process, but back to the 10.15.0.

There are many blog posts and guides regarding what to do when you go through the more classical path of having an update be downloaded to /Applications/Install macOS xxx.app.

My guess is that at some point, both ways of upgrading follow the same process (eg reboot to some kind of "upgrade partition" that will overwrite the OS files, reboot to upgraded OS). I would like clarifications on the parts where these upgrade paths differ, so that I can troubleshoot what is preventing me from reaching 10.15.4.

In the past, I did similar things with 10.14 and my notes on the topic don't show such an issue (rather issues with lack of APFS support in VirtualBox at that time). In particular, I think the partition layout has changed since then.

Essentially, what I tried so far:

  • start manual upgrade process through the .pkg, reboot
  • interrupt boot by hammering Esc, reach some kind of grey and blue boot manager (is that VBox or Apple stuff? this is not the VBox EFI shell, there is a text-based UI menu with "Device Manager"/"Boot Manager"/"Boot Maintenance Manager")
  • explore partitions, looking for an EFI boot file (I spotted the recovery one, can't find the "regular" one nor the "upgrade" one)
  • boot

For example, I wonder if the pkg is partially extracted and that I am supposed to find a boot.efi somewhere on the "System Data" partition. If so, where should I look for it? Do I have a more "sysadmin" way of deploying this upgrade, in particular without reboot?

  • In my experience, macOS updates are extracted to an "install sandbox", the system is sent into a special mode where the updated files are "shoved" over the existing installation (then any post install steps), then a reboot. There shouldn't be any separate partitions or EFI boot files. This may have changed with the separate system partition for 10.15 and later, though. In the past, I've have combo .pkg updates not apply after the restart, requiring me to install it again. May 26, 2021 at 23:29
  • The combo update is over 4GB. The MacMini is mostly idle, the storage is fast, but the upgrade inside the VM feels too quick to actually have done anything to the current OS, besides preparing everything for an actual installation upon reboot (where I expect to spend a few minutes). Do you have any idea where to look for this "install sandbox"? I'm going to dig inside the content of the pkg itself.
    – slv
    May 26, 2021 at 23:38
  • It's probably in /private/var/folder/zz/... or /.PKInstallSandboxManager. But you should easily figure it out by starting to install it, then pressing CMD-L to show you the log. Select "Show All" and it will spit out where it's extracting it to. May 26, 2021 at 23:43

1 Answer 1


I restarted from scratch, with a new dynamic VDI of 60GB instead of 30GB, as I felt I would be in trouble later with the installation of Xcode.

The upgrade went flawlessly.

There was definitely no obvious sign nor warning that more space was needed previously.

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