Malcolm Rayfield wrote:
Starting with Option pressed looks on each drive for a bootable system, so it knows which drives icons to display. A severely corrupted drive could make this process fail, and not allow you to boot from any drive.
This should never happen because of a corrupted drive: the BootROM of the Mac is responsible for selecting which operating system to run from some partition. It is a part of the Mac's hardware, not something loaded from a hard drive or other storage media. (This is the main reason for its primitive user interface.)
The storage media supplies the boot loader code that actually loads the OS, so drive problems can prevent the OS from loading or its partition from showing up in the Startup Manager, but no problem on one drive should prevent other boot sources from displaying their icons if they contain an OS capable of booting that Mac.
What can affect the Startup Manager is corrupted PRAM or fundamental hardware problems, like power management issues. This is why resetting PRAM or the
System Management Controller (SMC), & so on are recommended procedures for this type of problem.
Should you be curious about the technical details of the boot process,
this Apple developer document is a good place to start.