The alternative is to keep old versions in Virtual Machines in Parallels Desktop. SATA/USB-C disks may need to be connected to a USB-A port rather than USB-C.

Some external disks can only boot when physically connected in certain ways, e.g.

M1 Macs can’t boot from an older release of Big Sur without setting that bootable volume group to Reduced Security using Startup Security Utility in recoveryOS.

This currently appears insoluble, making it essential to create each bootable system when that version of Big Sur is still current.

Full installers for older versions of Big Sur don’t work when run in a later release.

M1 Macs pose several problems for doing this:

This article explores how successful making full clones can be, specifically whether you can use them to assemble a multi-boot disk containing multiple versions of Big Sur.ĭevelopers, researchers and system administrators often keep a collection of previous bootable systems so they can go back and run tests under those older versions, for instance when someone reports a bug which can’t be reproduced in the current macOS. The latest version 6 of Carbon Copy Cloner can now use macOS tools to make a full clone of a Big Sur System Volume Group, although this is no longer recommended.

Until recently, the only reliable way to create an external bootable disk on an M1 Mac has been to install Big Sur on it, either in recoveryOS or using the full Installer app.

Defragment macbook air