TeraByte Unlimited

I copy/pasted OS1, creating OS2 & OS3 [Windows]. Forgot to check "Add to boot menu" so I manually setup OS2 and OS3 to boot in "boot menu" by pointing to the boot partition and boot file. I found if I point a second OS to the "EFI System partition", it hangs on booting at the first Windows icon. If I create another copy and rename it, and then point the new OS to this new EFI System partition, it works without the hang. This is true even with 2 identical OS partitions.
Any explanation why it seems I cannot use the same EFI System partition for all or multiple Windows partitions?
Does booting change anything in this partition?

You would use the same EFI System partition for the multiple Windows installations. Using the "Add to boot menu" option does just that. Creating/copying multiple EFI System partitions would cause issues. You would need to remove those and setup booting from the single EFI System partition (the one with BIU installed to it). You can use the fixboot script to add the booting files to BIU and then edit the boot menu items to point to them.

When BIU was installed it should have been installed to manage the directories under the EFI folder (this is the default). Also, each BIU boot menu item needs to have the "Rename Directory" option enabled. This allows using the Microsoft.001, Microsoft.002, etc. folders.

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As a note regarding the other issue you posted about, if you did copy the EFI System partition that contained BIU then it would be on the drive multiple times (and is not a supported scenario).

Ran fixboot. I see no new options in boot menu. Rename is greyed out, cannot check it.

If the rename option is not available then the option to have BIU manage the directories wasn't enabled. You can check that by going into Partition Work, selecting the BIU install's EFI System partition, clicking the Properties button, scrolling the Flags section down to "BIU Managed" (bit 55) and enabling it. If this isn't enabled there will be issues with multiple operating systems using the same files (as Windows does with the contents of the EFI\Microsoft folder).

You didn't say if you actually have multiple EFI System partitions on the drive. If so, until the copies are removed they're going to cause issues. There should only be one EFI System partition on the drive.

Since all is booting now with separate EFI partitions, would it not be prudent to not kick the sleeping dog and leave as is?
What would change to stop the working booting processes?

I haven't tried that configuration myself. However, if it's working and you don't want to disturb it feel free to leave it be. Just be aware that there may be issues that arise that wouldn't normally. For example, if Windows updates or gets confused then it may change something on one of the partitions that doesn't belong to it (would likely break booting, but fixable). Some tools, scripts, etc. may not work properly if they only expect one partition. Also, I would only have BIU on the main one and not on any of the copies.

With how you have it configured now, are you hiding the other partitions from the one you're booting? Or are they all visible to each Windows?

I am hiding all partitions except the working OS.

I figured you probably were. I would keep it that way.

What does the rename option do?

It allows BIU to manage the boot folders by renaming them. For example, \EFI\Microsoft.001, \EFI\Microsoft.002, etc. Then when the .001 Windows is booted it will rename that folder to \EFI\Microsoft so Windows sees it normally as expected. In your case, with multiple EFI System partitions, it wouldn't be used or necessary since each Windows has its own partition.