Setting a default Startup Disk

nydesign wrote:

I added an external SSD, installed on it Mac OS Ventura, set it as my Startup Disk and it successfully started from the SSD. But when I shut down and restarted, it started up from the Mac HD, not the SSD. How do I set the SSD as the default Startup Disk, w/o needing to designate it repeatedly?

You do not say what exact iMac...

see Intel T2

About Startup Security Utility on a Mac with the Apple T2 ...

Mac models with the Apple T2 Security Chip

M1/M2—

How to boot macOS on an Apple Silicon Mac from an external bootable drive

1. With the Mac powered off, connect the external bootable drive to the Thunderbolt 3 port.
2. Turn the Mac back on with a long press of the power button, holding until the screen shows startup options.
3. Select the bootable external drive.
4. Your Mac will then boot from the external drive instead of the internal storage.

You could set the Mac to boot just from

the external drive without even considering the internal version. This

can be controlled within macOS itself.

How to set which drive an Apple Silicon Mac boots from

1. In macOS, open System Preferences and select Startup Disk.
2. Select the drive you want to boot from.
3. Click Restart

ref: https://appleinsider.com/articles/21/01/03/how-to-boot-an-apple-silicon-mac-from-an-external-drive