You are likely going to need a third-party app to handle your booting, since you installed the older OS after the newer one.
Or, as a better suggestion, repair your Windows 10 so it boots, then virtualize Windows 7. Very little, if any reason to multi-boot anymore, and it's much safer for your overall system.
Try this - it usually works in this sort of case -
Install Macrium Reflect Free (on another pc if necessary).
Create Macrium Rescue USB flash drive.
Boot from it and select "fix windows boot problems"
With luck, it will detect both OSs and rebuild boot files and boot ok.
The problem is that you have the Windows 10 SSD which is GPT partitioned and using UEFI to boot. Then you have the Windows 7 HDD which is MBR partitioned and using legacy BIOS mode (CSM) to boot. So not only do you have to select which physical drive to boot from, but you also have to change the boot mode between UEFI and CSM (legacy BIOS mode). You need to convert the Windows 10 SSD to MBR and set it up for legacy BIOS mode (CSM) booting. That means making a backup image of the Windows 10 OS partition only, convert the SSD to MBR, restore the Windows 10 image back to it, and then add the legacy BIOS mode boot files to it.
In order to boot Windows 10 now without changing anything you need to go back and set the UEFI firmware to boot it in UEFI mode - the error you are getting when you try to boot Windows 10 is because you are trying to boot it in legacy BIOS (CSM) mode which is what was required for you to boot the Windows 7.
The changing drive letters C: to E: and D: to C: is normal when dual booting. The OS, when it boots, will always give itself C: by default and assign other drive letters to the rest of the drives/partitions.
What i did today: I Converted my ssd Into mbr and cleaned it via diskpart.
I installed Windows 10 and Set it up (installed my Programs etc. ).
I didnt try to Boot from my second Drive (Win7) because I dont want everything to be messed Up again. Should i Go for it and Just try to Boot or will it **** Up my PC again?
If you want to set up real dual booting:
In Windows 10, your Windows 7 partition should have a drive letter - let's say E:. Open a Command Prompt (Admin) or Powershell (Admin) and run:
bcdboot E:\Windows /addlast
Change the E: to whatever Windows 7 is. Then restart the computer and you should have a dual boot menu.
I tried that and guess what, windows 7 doesnt start anymore. It Shows the same Error as Windows 10 did when I tried to Boot it after installing Windows 7 (something with a corrupted .exe). Im gonna stay With the virtualization because I think dual booting Just doesnt Work Out for me. Could Tell me the Command, to remove Windows 7 again?
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