EFI BOOT, Forbidden Sign, EFI Firmware update - USB Boot won't load

Hi Guys, I've got a weird one here so hopefully someone will have experienced the same issue.

Basically I have 2 MacBook Pro's (13" & 15") early 2011 and I have decided to upgrade both with some extra RAM + SSD Drive.

I managed to create an USB Boot disk (High Sierra) and proceeded to try and boot the 15" MacBook with it but after selecting my USB drive, the loading bar would load half way through and the I'd get a Forbidden sign. I did some research and found out about the issue with 2011 MBP's and about upgrading the EFI firmware in order to be able to boot the system using my Usb (which I have done through security update after re installing high Sierra onto Hard Drive). Eventually, I managed to upgrade EFI firmware, then went and removed the hard drive, put in my new SSD, boot it with my USB and completed a fresh install of High Sierra on new SSD. Everything was fine. Easy done!!!!

Now, when I went to do the same thing with my MacBook Pro 2011 13", which was running Sierra, I came across this issue. I upgraded the MacOS from Sierra to High Sierra (using hard drive) then once logged in, I downloaded and installed high sierra's security update, which contains the EFI Firmware update, but here's when it gets tricky and weird. During the reboot of system and install of update, my computer wasn't plugged in to power charge, and the Apple Logo screen for the EFI Firmware updating loading bar kept restarting the system, probably 3x. I tried to run upstairs to find grab my charger but it was too late. Eventually the installation was finalised but I could tell that Firmware wasn't fully updated because the MacBook wasn't plugged in to power source. So I then proceeded to remove the HD, put in a brand new SSD and tried to boot it through my High Sierra USB boot to do a fresh install onto the new SSD. Again, booting through USB was forbidden. So AGAIN, I put the Hard Drive back in, went through recovery mode, fresh installed High Sierra again, ran software update, which installed Security Update 2020-002 again. This time I made sure to have my MacBook plugged in during the restart and installation, but I accidentally left the High Sierra USB drive plugged in during this process. Everything went smooth, Firmware update screen was completed this time and after completion,I tried to boot it through USB (option key at startup) and this time It would show: my Mac Hard Drive, an EFI Boot Hard drive (which I've never seen before), and my High Sierra USB boot disk was now showing up as "EFI BOOT" instead of whatever name I had previously saved it as the High Sierra Installer. And obviously, it wouldn't boot through my USB.

So basically, as weird as it sounds, I'm thinking something my have happened during the security update and firmware update install (wether it was because of not having the computer plugged in or leaving the USB stick connected.)

I then tried to boot my MBP 15" with my USB just just to check if there was anything wrong with my USB installer, and Boom, the USB was now showing up as "EFI Boot and I wasn't able to boot it.

I boot the system through the SSD, and when I clicked on the High Sierra Usb installer, it had been deleted off the USB drive. So now, the USB stick won't work even in the MacBook it had previously worked fine. It was so weird!!!

I don't know if all this makes sense and I hope someone would share some light into this.

So basically, Is there anyways I could re install this Firmware update, or any other suggestions?

All I want to do, is install High Sierra into the new SSD and plug it in. But its been such a mission. I never had any issues with any other MacBook I upgraded to SSD other than these two 2011 models. Very frustrating.

Many thanks,

Daniel