Making an external bootable SSD, 2019 27 iMac, security settings?

yes, i misunderstood your question. we're on the same page now.

Saint Martin wrote:

Just the top portion, not the Secure boot and External boot options.

the secure boot and external boot selectable options are only available on macs with the T2 chip installed. in startup security utility, only the firmware password protection option is available on machines that were made before the T2 chip was introduced. that's why i didn't understand your question until i read P. Philips' response. my machine also pre-dates the T2 chip.

i'm not sure if this is what you're asking. but you open System Preferences > Startup Disk and choose which drive you want to use as your startup disk.

if you need to use the other disk, restart the mac and after the startup chime, press and hold the "option" key until you see the choices of startup disk. then when done using that disk, just restart the mac and it will boot into the disk chosen in "startup disk".

You mean to say the image below does Not Appear ?

This is what should be presented