24-inch iMac (M4) not booting from external drives

I had tried using SuperDuier and Carbon Copy Cloner to create an external boot drive from which to boot. Neither worked. When I contacted Carbon Copy Cloner developers they told me the new Silicon Macs can't be cloned. They have to be created in the following manner:

CCC's legacy works fine with Intel Macs but not with Silicon Macs.  The following is what Rob of Bombich Software told me as a way to clone my internal boot drive on a Silicon Mac to an external SSD:

1 - Boot into Recovery Mode

2 - Erase the external disk in Disk Utility

3 - Install macOS on the external disk

When the installation has completed, accept the offer to migrate data from your original source disk or your Time Machine backup.  

It worked for me. Just keep both drives up to date on system updates so that if any of the updates include a firmware update both will be on the same version.

On Apple Silicon (M4) Macs, exact "block-by-block" clones made with SuperDuper! no longer work the way they did on older Intel Macs. For security reasons:

1. The operating system must be cryptographically signed by Apple (Signed System Volume).

2. The internal chip (Secure Enclave) must authorize and "take ownership" of the external drive.

The installer you ran from the desktop skipped this process because it detected the drive was already "full."

The 3-Step Solution

1. Wipe the drive: Open Disk Utility, select Show All Devices, click on the root of your external SSD, and erase it as APFS with a GUID Partition Map.

2. Install macOS from Recovery: Shut down your Mac. Press and hold the power button to access startup options, select Options (Recovery Mode), and run Reinstall macOS Tahoe, choosing your clean external SSD as the destination. This will properly configure the M4 boot permissions.

3. Migrate your data: Upon reboot, the Setup Assistant on the external drive will ask if you want to transfer data. Choose to do so from your internal Macintosh HD to copy over your users, apps, and files.

Moving Forward

 Backups: In the future, use SuperDuper! to update only the Data volume, not the operating system itself.

 Hardware Limitation: If your iMac's logic board or internal storage suffers a total hardware failure, the M4 will not boot at all—not even from an external drive—as the core boot firmware always resides internally.

Here is an Apple article for creating a bootable external USB drive (very important detail at the beginning of the document):

How to use an external storage device as a Mac startup disk - Apple Support

Bootable clones are a thing of the past on Apple Silicon Mac due to numerous changes to macOS security & hardware. Sometimes you may be successful for a while, but eventually you are likely to discover a problem when attempting to boot or update an external macOS boot drive. The developer of Carbon Copy Cloner some years ago posted the unfortunate news after Big Sur was released. Plus Apple Silicon Macs have a new concept of ownership which can end up causing problems with the system in general, much less for a bootable external drive.

A few times I have installed macOS to another APFS volume on the internal SSD to assist in troubleshooting. Afterwards I when I try to reconfigure the default Startup Disk to the original boot volume I find that it will no longer boot and insists on having macOS reinstalled (requires a newer patch level as it won't accept reinstalling the exact same patch level). I have suspicions it is due to a pending software update patch on the original OS that was staged, but never installed and booting from another OS installation confuses the system. It is not always possible to save that original installation.

In addition, if the internal SSD fails on an Apple Silicon Mac, then that Apple Silicon Mac is a brick until the Logic Board is replaced. There are required system files on the internal SSD which are needed to boot into Startup Options (aka recovery mode) and to external boot drives.

I agree with Old Toad. You can also use CCC to first clone your data to the external then boot into recovery and install the OS on that external drive. Either way worked for me.