Mac Pro 5,1 won't open Boot menu

I thought I had successfully installed High Sierra on sata ssd, (via MBP). Connected it up in MP but still couldn't boot into it. No key command options worked. After a couple of days of trying Command+R my MP finally booted into the external CCC backup - I put the drive into a different usb slot and it worked... it had worked in previous slot before.

Installed OS on HD from backup... I hoped to boot up in recover mode and then do clean HS install to ssd from usb installer. but MP still wouldn't boot.

The issue turned out to be the ssd formatting. When erasing drive I had used APFS, it should have been macOS Extended. Although HS uses APFS, the MP 5,1 shipped with Sierra - which is macOS Extended. Now, with the reformatted ssd I booted into Recovery Mode and from there reinstalled Sierra from Apple. So all ended well...

I've never come across an intermittent USB connection before, (CCC Backup drive), but it certainly muddied the waters when trying to diagnose where my problem lay - two unconnected issues occurring simultaneously...

Thanks for the trouble you've taken to steer me through all of this, much appreciated... I've now got a clean Mac Pro macOS Sierra I can put up for sale.

I was thinking if I installed HS from CCC on another internal drive, I could get into Start up disc. Then select newly created HS drive.

HI there,

As I'm planning to sell my Mac Pro I've gone the recovery mode route, erasing drive disc in Disk Utility. The problem arises when I choose 'reinstall macOS', and the installer pain comes up saying the firmware needs updating and requests shut down... but the 'shut down' command is greyed out, so I can't update the firmware. I've read a few posts on the same problem, that's why I tried installing from a USB installer - using power/alt... but the computer won't boot. My final route was to create a installation on another Mac, which didn't work out.

I am using a wired keyboard

Thanks for that link but unfortunately my Mac is no longer booting so I can't get into Terminal. I've done a thorough nvram reset - 4 tones.

Yesterday it ignored recovery mode Command+R but opened High Sierra OK. Today nothing is happening.

Sorry for delay in replying, first chance I've had to check things out.

Changed battery but still not opening in recovery... although the Mac did boot into installed High Sierra, which it had stopped doing before battery change, so was able to enter the Terminal command from your link, 'sudo nvram manufacturing-enter-picker=true', but I'm getting message 'command not found'.

I had created a USB High Sierra installer which I connected and shows up in disk utility, is there a terminal command that will instigate installation from USB installer.

I'm not sure I understand how using the terminal command to open apps as outlined in your link would help me in my dilemma.

Reading my last post I realise it wasn't clear exactly what I was wanting to achieve.

If I connect my Installer macOS High Sierra USB drive to my Macbook Pro could I install a complete HS installation on an attached external sata ssd, with the view to slot into the Mac Pro and boot up from that?

Yes that's I want to do. Tried this before but wouldn't boot up when connected to MP so thought the method I had used hadn't work... when I connected the drive to MBP the get info showed the memory used on the HD, (can't remember exact figure), but it seemed too low to be a complete macOS.

I've tried as you suggested again but MP not booting to it. I reset nvram, then straight power on. Then command+R and Alt at boot up, with no success.

Yesterday I was able to boot to a Carbon Clone backup external drive and run HS.. not working today - this happened before. Maybe I need to leave it for a bit - after all the various startup attempts I've tried.

Thanks for that, I'll look into it.

Sorry, should have said, it's a 2012 MBP running HS I would work off.