Can I safely install Windows 10 on the entire internal SSD on a T2-based Mac (mini) rather than splitting the drive space between macOS and Windows? Is there a particular method for this that I should use?

Longer version:

I have a 2018 Mac mini. I opted for a model with a small internal SSD as Apple's SSD pricing is ridiculous. (I also upgraded the RAM to 32GB myself for the same reason.) After some wrangling with the T2's secure boot peculiarities (it seems you have to install macOS from the recovery environment; running the installer app from the App Store results in an OS installation that this Mac won't boot from.), I'm now using macOS from a 2TB USB 3.1 external SSD instead, which cost less than 1/5 of what Apple would have charged me for that capacity.

I'd like to occasionally boot Windows on this Mac as well. From past experience, attempting to boot Windows from a USB drive is not worth the trouble, and Thunderbolt drives aren't entirely straightforward for this purpose either. Plus, they're comparatively expensive. I'm not using that internal SSD at the moment though, so that seems a good place to keep Windows.

I've read a few horror stories of people apparently bricking their T2-based Macs by wiping the internal SSD though; keeping a small macOS installation around on the internal drive and only using part of the SSD for Windows is certainly possible, but it'd be much nicer to just use the whole thing for Windows and not constantly be fighting for disk space.

Is that possible? The Boot Camp assistant obviously won't let me do it directly, but can I safely wipe the drive from Windows setup and use the whole thing for Windows anyway without the T2 going into lockdown mode? Or should I be going about this some other way?

Note: Based on the responses so far, I feel I should point out that I'm definitely not a Mac newbie, I'm familiar with how I would do various Boot Camp, partitioning and OS setup tasks on older Macs. (And I happen to write Mac device drivers for a living.) The T2 security chip in the 2018 Macs and the iMac Pro significantly complicates matters of booting, even with secure boot disabled. I'm looking for answers about T2 based Macs specifically, from people who have practical experience with such Macs.

  • Through macOS, you can turn off the secure boot functions of the T2 chip. This does seem like a very expensive path to get a (slower by way of USB boot drive) Mac and a Windows machine, however. – Allan Jan 4 '19 at 13:19
  • @Allan Yes, the T2 secure boot is off on this machine. macOS is not noticeably slower apart from early boot, but that's rare enough that I don't care about that. I don't understand the "expensive" comment. Either way it doesn't answer the question. – pmdj Jan 4 '19 at 13:34
  • Which is why I posted it as a comment.... Have you attempted to install macOS once you turn off secure boot? – Allan Jan 4 '19 at 13:36
  • @Allan macOS is working after installing from the recovery environment, yes. As I said, it did not work when installed from the App Store app, and yes, all the secure boot setting were set to the least secure. (Depending on the various different ways you can do the install, you either see the Apple logo and progress bar but that aborts and the system reboots, or you get a message saying the OS needs to be updated before it can run on this Mac; this is despite the fact that it's macOS 10.14.2. On an older Mac it works fine.) Anyway, the external macOS is working, I'm asking about Windows. – pmdj Jan 4 '19 at 13:37
  • When originally trying to solve the external macOS issue, I found that a number of other users had the same problem on other T2-based Macs, with the same eventual resolution of using the recovery environment, so although secure boot is in the least-secure setting, these Macs seem to be a lot more fussy about what they will and will not boot. Indeed, supposedly if you run an OS other than Windows or macOS, you will not have access to the internal SSD at all due to the wrong boot loader signature. So Linux etc. will only work off external drives. (NB: I have not tried this myself.) – pmdj Jan 4 '19 at 13:46

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