startup – Why am I unable to boot macOS from an external device on MacBook Pro M3?

I’m trying to install macOS 15.0 (24A335) on an external memory device and boot from it on my MacBook Pro with the Apple Silicon M3 chip that runs the same build on the internal SSD. Here’s what I’ve done so far:

1. Initialized the external device with a GUID partition map
2. Created an APFS partition on the device
3. Downloaded the macOS 15.0 (24A335) installer directly from Apple’s SWCDN
4. Ran the InstallAssistant.pkg and obtained a copy of Install macOS
5. Launched Install macOS and installed the OS on the external device

Everything went smoothly up to this point. However, after the installation:

• I tried selecting my external device as the startup disk and restarted the computer, but I received the error message “The version of macOS on the selected disk needs to be reinstalled.”
• I booted into recovery mode, and my external device appeared in the list of bootable disks. But when I selected it to boot, after some initial loading, nothing happened.
• I also selected the device as the startup disk from recovery mode, but again got the same “The version of macOS on the selected disk needs to be reinstalled.”
• I made sure that my Startup Security Utility settings are set to “Reduced Security” and that the device’s information on Disk Utility shows “bootable: yes”

I’ve previously installed and booted countless operating systems on my older Macs and PCs with no issues, from unsupported macOSs to Windows and Linux distros, thanks to the abundance of forums and guides online. This time, I’m struggling to find anyone with a similar issue, and there’s very little information available. One website briefly mentions that “It’s not as easy as it used to be, as it likely requires you purchase new hardware”, but it seems to refer to needing a USB 3.1 or higher external device, which I tested with a USB 3.2 flash drive without success. Another blog even provides a tutorial on “How to Boot Intel/T2/M1/M2/M3 Mac from External Drive”, which suggests this should be possible, but I haven’t had any luck. I read some other things around, such as naming the device with no spaces and changing the USB port to plug it into, but nothing worked.

I’d prefer to boot macOS from an SD card in the built-in SD reader (for minimal clutter, even if it’ll be slow), but I’m open to buying a new USB or Thunderbolt device if it guarantees success.

Has anyone encountered a similar problem? Any suggestions on how to resolve this?