Is it possible to boot a Sonoma MacBook Air M2 from a Sequoia partition on an external SSD?

FWIW, you should always use a dedicated drive for Time Machine. If you do install Sequoia on the external drive and it fails, you will lose everything. Just because you can create multiple partitions doesn't mean it's always a good idea.

UnderwaterSunlight wrote:

I use a MacBook Air M2 2022 with Sonoma 14.7.6.

I have an external LaCie SSD 2TB USB-C on an OWC Thunderbolt4 dock. One of this SSD's three partitions has 400GB and is formatted APFS (completely empty). To this partition I want to install Sequoia.

I read it is feasible to boot from an external SSD as a whole, but is it possible only to boot from the aforementioned partition?

Yes, you can install Sequioa to an APFS volume on an external drive.

Is the partition in this case acting like an entirely separate drive?

Yes and no. Some of the boot files required for an M-series Mac always resides on the internal SSD. In fact the internal SSD of an M-series Mac will always have the preboot files for the highest OS that is installed on the system (internal or external....those preboot files are shared).

If anything happens to those internal preboot files or the internal SSD fails, then you will not be booting an M-series Mac at all. Those preboot files on the internal SSD of an M-series Mac used to be within the system firmware on the Intel Macs.

Will this partition successfully show up as a bootable source in Startup Options?

Yes.

I am asking even before I install Sequoia to this partition because I want to get it right from the start.

Can someone please advise me?

What are the other two partitions on the external drive?

FYI, I never recommend anyone partition any drive because they usually find out much later that one or more partitions are too small.

When using the APFS file system on any drive, you can create new APFS volumes which act like a partition without requiring dividing up the physical space into discrete sizes. Instead each APFS volume within the Container shares the storage pool with each of the other volumes. Each APFS volume can be mounted & unmounted independently and can even be encrypted individually.

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support

Note that these days, Mac startup disks have a rather complicated internal structure.

This is what "Macintosh HD" (in Finder) looks like internally – not even counting "VM", "Preboot" and "Recovery" areas that are only mentioned in passing if you click on "Container disk3". The second "Macintosh HD" here is a signed, sealed system volume; and the system runs off a read-only startup-time snapshot of it. All user files, and macOS files that might need to be modified during normal operation, live in the Data volume.

An APFS container is similar to a partition except that all volumes within the container share a common pool of free space. The APFS volume group would appear to serve the purpose of binding together several individual volumes into something that the Finder treats as a single "drive".