Your experience may vary of course and even if this does seem to be possible across current devices I would expect this is likely a boot loophole which may well be closed via firmware updates sooner rather than later. This did enable us to install and provision Mojave via our usual workflow using the startosinstall command. This was even with the startup security settings untouched! This was a surprise as I didn't expect USB storage devices to be scanned by any boot sequence of T2 equipped machines with boot security set at the default level. I found that a recent T2 equipped MacBook Pro we tested could in fact be booted from a Mojave recovery partition on a USB drive by pressing cmd+R after the local drive had been wiped entirely via the disk utility of the Catalina recovery environment. IR is an option on these as Mojave 'should' be the IR version presented but obviously this looks like its going to be an issue when we receive a device that shipped with Catalina and the IR is Catalina too. Devices that shipped with Mojave and have upgraded to Catalina cannot be wiped and reloaded from the Catalina recovery. In testing I've found it doesn't seem to matter what OS the device shipped with, just that you will need to get into a recovery boot session of the macOS version you want to install (or earlier). Our organisation has yet to receive any devices that shipped with Catalina just yet but it is only a matter of time. It just looks like this isn't being well communicated in this particular combination. I'm sure I've seen a response from the startosinstall command alluding to something like that in the past. I seem to recall in the past that attempting to 'downgrade' from the recovery boot of one version of macOS (e.g. Has anyone successfully installed Mojave using a startosinstall command from a Catalina recovery (or Internet Recovery) boot? I'm seeing the same symptoms of it just hanging at "Preparing to run macOS Installer." It does not sound like your issuee was like mine but I thought I would share this here. "Īpple sent me this solution and it worked on one and I am running through the other: In other words, you need to use Configurator so that we can custom-build and install firmware that will be specific to your system and yours alone - this customized system-specific firmware is what was destroyed when the drive was formatted, and what will need to be replaced in order for you to reinstall the operating system. The firmware on T2 equipped systems is uniquely customized on a per-computer basis. This is due to how the T2 chip and secure boot firmware function, and because formatting the HD wipes the firmware that is not just hardware-specific. Here is Apple support explanation: " T2-equipped systems like MacBook Air 2018 require Configurator 2 and a 2nd T2-equipped system in order to reinstall the firmware and operating system once the HD has been formatted. The reason was b/c we erased the firmware when support formated the system with disk utility. The laptops would act like they were building and then loop back into recovery. For me on 2 MacBook Airs the recovery would not work at all. I reached out to Apple support we are fortunate to have enterprise and I received this on how to fix it. I found out about the T2 devices after erasing and rebuilding on an older system, pre-firmware boot laptop that did originally ship with High Sierra.
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