I would rather try using the EasyBCD software, because I'm not very familiar with cmd, I could try... but what if something goes wrong?
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I understand now why you suggested me that, is it maybe because I already have the "bootmgr" file on my C: SSD's partition?
And after running that cmd, what should happen and what should I do afterwards if my PC boots normally?
Sorry but I'm very paranoid, what should I exactly do after pasting the command into the cmd and pressing enter, should I shut down my PC and then unplug HDD and see if it's booting normally then, or should I restart my PC and shut it down afterwards and then unplug the HDD and boot it again.
Again, sorry for me being so paranoid, this is my first time doing something of this kind.
Run the command. Watch for the message that it completed successfully. Shut down Windows fully by holding down the shift key when you click on Shutdown from the power menu. Disconnect the HDD. Turn the computer back on.
If it boots fine with HDD disconnected, then you can shut it down again, and when you reboot, you can do whatever you want with the HDD. The bcdboot command will do absolutely nothing to the HDD.
There's a bit more complicated way.
Install Macrium Reflect free Macrium Software | Macrium Reflect Free (you are going to need it anyway). let it make a rescue USB, boot from it and choose "Fix BOOT problems". It will ask you on which disk and tell it to fix SSD. Disconnect HDD and it should work. You can connect it later and clean it up or do whatever you want with it.
Alright, I will try NavyLCDR's solution, I will post my BIOS settings here just in case if I need to change something there (album Imgur: The magic of the Internet)
After reading this I can offer my experience, an older computer with 2 HDDs in it, had an issue with changing from Home to Pro but got through that okay. Then I decided to remove the D: drive and the computer wouldn't boot, in checking the BIOS found it had a setting for AHCI/RAID that Windows had picked up the entry and set up RAID with no notice to me, removing the drive broke the boot process. Quickest/cleanest solution was simply reinstall Win10 on the only drive in the system, fortunately it was one of my Insider Preview machines so I lost nothing in the way of my data. That computer had been running the IP for some time with only one issue, had to use an Add-in video card as there are no drivers for the onboard adapter for the lastest few Upgrades.
Let's see what happens after the restart https://i.imgur.com/olkDII5.png
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I did it, but unfortunately, I got the same message after I unplugged the HDD (precisely, this message: https://i.imgur.com/wmw2QQj.png)
After I got that message, I shutted down my PC once again, and I went into BIOS (here are the BIOS settings after performing your cmd (album Imgur: The magic of the Internet)
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After performing the cmd command, nothing changed in the Disk Management either https://i.imgur.com/7Xf0Sbc.png
I can not add anything to the problem but:
Untill I did this same drive dual boot option I was using power switches to my hard drives to swap O.S. systems & had to reset bios at starup once i had switched hard drives as it was looking for the other drive which I assume has a different id code to the one which was stored on the hard drive after switching.
You could use a swap caddy to change drives if you can fit one in your case to change O.S. systems.
FYI - I did this on my X570 2020 motherboard.
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