How to force Startup Manager menu to appear at every boot?
Solution 1:
I'm not sure this is possible by default without holding down the ⌥ key at startup. You can make the Boot Camp partition always boot by selecting it in System Preferences → Startup Disk, but it sounds like this isn't what you want.
However, if you're not adverse to installing additional software, I think rEFIt may do what you need. The section on Getting into the rEFIt menu says:
If you have installed rEFIt on your Mac OS X volume, it will be loaded automatically when the Mac starts up.
You should then be able to select whichever OS you want to boot from there.
Solution 2:
You don't without modifying things on a low level or adding custom software like the awesome Boot Runner. When a mac can't find the first viable image to boot, it's programmed to get the prohibitory icon - not the boot picker.
Intel Macs have the following basic behavior at boot time when the chime indicates a successful POST. This is the hardware behavior and unless you rewrite the code / replace the hardware, you are bound by this functionality.
- read the value of the startup disk from NVRAM
- try to find that volume
- launch mach_kernel (or EFI on Windows / Linux side of things) and exit
- if that volume is not present (or NVRAM is reset to defaults), find the first viable boot volume in the local device tree and hand things off to mach_kernel (et. al.) so the boot process can exit
The only deviations from this are when you press one of the startup key combinations to alter this list.
- Startup key combinations for Intel-based Macs
So, you can:
- press a key each time
- introduce only the volume you want to boot (which bypasses the screen you wanted to see - but might get you the end result)
- set an alternate bootable image like rEFIt in NVRAM
- get into hardware / firmware hacking to make this the default
I don't know of a good guide to the last bullet point, but internet fame awaits someone (perhaps you!) that figures a nice way to hack a mac to show the boot screen as you ask.