What appears to be a bug in macOS 27 is preventing a number of users from selecting alternate partitions or drives to boot from. This is having a big impact on Asahi Linux users.

Most Mac users won’t ever have to deal with issues such as switching the partition their hardware boots from. However, for a contingent of users who use multiple operating systems on their Mac, there seems to be an issue with macOS 27.

In a post to Mastodon, Asahi Linux has warned that Apple has changed how the boot picker and Startup Disk applications detect if there are valid OS boot volumes available.

Users of Asahi Linux who update their Mac to use the developer beta of macOS 27 Golden Gate may not be able to see the Linux partition.

Asahi Linux explains that the partition still exists on Macs, and that no data has been lost. It is just a case of the partition not being detected properly, meaning the Mac won’t switch and boot from it.

For users who have updated to macOS 27 and happen to have a secondary installation of macOS 26 handy, such as on an external disk, they should set macOS 26 to be the default Startup Disk. Doing so will restore access to the Asahi Linux partition.

At the moment, the Asahi team won’t be supporting users who have installed the macOS 27 beta if they don’t make contingency plans.

While the issue has been publicized by a prominent Apple Silicon Linux project, it is something that has the potential to be a wider problem for multi-OS users. Early AppleInsider testing proves that users of older macOS versions on separate partitions will encounter similar problems.

A fixable early issue

The boot picker is effectively a macOS app that runs in the recovery environment of the default boot volume. The installation of macOS upgrades can affect the recovery environment and its included software.

The Asahi developers believe that this is a bug, and have filed a report with Apple, number FB22994760. The Asahi Installer has also been patched to prevent it from running on macOS until a workaround can be created, or if Apple fixes a bug.

Since this a problem found in the first developer beta of macOS 27, it may be an artifact of Apple dropping support for Intel hardware. The current version, macOS 26 Tahoe, is the last to work with Intel Macs.

With months more betas to go before the final release to the public this fall, Apple has a number of opportunities to get it right before that time.

Installer beware

The existence of a boot partition switching issue also serves as evidence for a common refrain AppleInsider repeats at this time of year.

While developer betas are tempting to install, don’t do so unless you’re prepared for bugs, issues, potentially bricked hardware, and other maladies.

Instead, mainstream Mac users should wait for the final fall release or the usual public beta track. It usually arrives a few generations into testing.

At the very least, wait for a few developer beta builds to come out, so that the bigger issues have been shaken out and fixed.