Win Bootmanager problems after installing 2nd Windows

Hello,
the following problem is solved, but it would very kind if someone could explain to me what happened.

I did the following:
1. Installed Win10 (reduced but doesn't matter i guess) on volume 0 (M.2 SSD). He wanted to create additional partitions for restoring, for system and another one.
2. Installed Win10 on volume 1 (HDD) with volume 0 connected to my computer. As far as I remember he now didn't ask to create additional partitions.
3. Disconnected volume 1

Then I wanted to boot from volume 0 as always, but he had a problem while booting with the file"C:\Windows\System32\winload.efi" (I don't know the exact error message anymore but the error came directly after selecting volume 0 for booting. Later I checked and the file was at least existing, but don't know if it was changed). Got the problem fixed more or less with the software EasyUEFI. I now just want to know what happend and why, so the Questions:

a: Why didn't he want to create additional partitions for volume 1 either?
b: Are the additional partitions essential for booting Windows if just this volume is connected?
c: Why did Windows touch my volume 0? I said he should install on volume 1.
d: How can I avoid the problem? Do I always have to disable/disconnect all other volumes when installing windows, or is there an easier way?

Many thanks and have a nice evening.

Hello,

I experienced these troubles few years ago.

a: because Windows detects all other Windows HDD installation system, and I assume if one partition already have the boot flag so no need to create another boot partitions (for uefi/efi at least)
b: for uefi/efi boot : yes .... but for mbr boot : no (it needs 2 or 3 manipulations but this could be easily done)
c: because the 1st windows installation detected is considered as the "master" installation
d: if you need all HDD with a complete boot partition, it seems you have no other choices to do as you described.

Hope this help

Thank you for your answer,

afterwards I can't exactly say what caused the troubles. Maybe I chose my USB device in non-UEFI mode (is shown 2 times in the boot device list). I think normally it should have worked in UEFI mode.

I found a good article about explaining UEFI, BIOS and the bootmanager which may help other newbies like me. Unfortunately it is written in german, but maybe it can still help one or two. Otherwise use google translator.
If links to other websites are unwanted, please tell me or delete it by yourself
[You must be registered and logged in to see this link.]

I absolutely love EasyBCD, the all in one Windows nt60 boot editor. The free version does everything you could ever ask of a boot editor, and its super easy to use.

But I can't post the link. Just google "EasyBCD", and you'll find it. I have been messing with it every day lately, I have at least 2 installations, and an extra partition I use to test installations on.

That will make you feel really stupid, it could have saved you all that trouble. I kept 2 hard drives in my desktop, both of which had good nt60 mbr's, and complete running installations, so either one can boot. I used EasyBCD to cross reference them, so each could boot the other in case something happened.I got (well, my roommate got it for me while I was at work) a nasty trojan that ate my nt60 boot sector, and left only an nt52 boot sector crying "ntldr is missing". Of course it is, nt60 uses "bootmgr". I just changed the boot disk in my bios, since each of my drives could boot the other, but only after I used my other windows to hunt that nasty bugger down and delete it. I still used it as an excuse to get rid of said roommate, and I never let on that my computer still worked fine. I didn't even change bios setting, it has a boot menu in the bios, I selected the other drive that way so said roommate wouldn't try messing with my computer while I was at work anymore. So when my best friend needed to keep his teenagers off the computer while he was out, I fixed up a usb boot disk with the bootmgr and bcd store on it, and deleted the bootmgr from his C: drive, and the usb drive was then the only way too boot the  thing, like a key. The uses for an easy to use bcd store editor are endless.