[Solved] A Tip rather than a question - resetting PRAM
I have just asked a question so to play fair I am providing a possible tip
I was trying to reset the PRAM on my late 2012 iMac running MACOS on an external USB SSD. The much slower internal spindle drive is now used for data storage and is not bootable. To reset PRAM this you have to hold down COMMAND-OPTION-P-R as you turn on the machine but before the chime
Initially I found that my bluetooth Apple keyboard won't work as I guess bluetooth devices don't get connected until later in the start process.
I thought that I had solved the problem by using my old USB Apple Keyboard BUT again the boot sequence defeated me as my USB keyboard was not being activated in time to reset the PRAM. I think that this may have also had something to do with the external MACOS boot SSD also being on USB.
Here is the tip.
Shutdown the iMac. Disconnect any external MACOS drives and all devices except the USB keyboard.
Now switch on. I was now able to hold COMMAND OPTION P R on my USB connected keyboard without any problems and reset the PRAM.
Hope it may help someone
Over the years I have often found the timing of the key press to be important - particularly for wireless keyboards - for most of the startup options. My assumption is that in some scenarios the keyboard needs to be "activated" enough to register the set of keys as being held down.
Anyway as an extra part to the tip here is the list of different startup keys & modes as pasted from Apple's support page:
Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up. If your Mac is using a firmware password, you're asked to enter the password.
Shift (⇧): Start up in safe mode.
N: Start up from a compatible NetBoot server, if available. To use the default boot image on the NetBoot server, hold down Option-N instead. Computers that have the Apple T2 chip don't support this startup key.
T: Start up in Target Disk Mode.
Command-V: Start up in verbose mode.
Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc.