How to Dual Boot Multiple HDD's

The command I had you entered is only supposed to add an entry to file that shows the Windows boot menu. I suspect there was some corruption in the Windows 10 file system and the bcdboot command writing to the bcd file must have triggered it. The computer could no longer read the Windows 10 boot files because of the file system corruption and fell back to the old boot files contained on the hard drive containing Windows 7. So let it boot into Windows 7. You don't have to do the next step, it is just a precaution, but I highly recommend it:

Create a bootable USB flash drive of Kyhi's Recovery Tools. This will have many utilities you can use to fix your system if it stops booting altogether:
Windows 10 Recovery Tools - Bootable Rescue Disk - Windows 10 Forums

The reliable way to make a bootable flash drive from the ISO file of Kyhi's Recovery Tools is:
Insert the flash drive with Windows running. It will be erased during this process. "Run as Administrator" Command Prompt. Run:
diskpart
list disk
select disk # <-replace # with the actual number of the flash drive
clean <- this will erase the disk selected above - make sure it is the USB flash drive!
create part pri
format fs=fat32 quick
active
assign
exit
exit

The flash drive should now have a drive letter. Mount the ISO file of Kyhi's Recovery Tools using a program such as WinCDEmu:
WinCDEmu - the easiest way to mount an ISO. And more...

Once you get the ISO file mounted, copy all the files and folders that appear to the USB flash drive. Then you can test booting to it to make sure you can get into Kyhi's Recovery Tools.

Set that aside for now. That's the end of the optional, recommended step.

In Windows 7 post a screenshot of disk management:
Disk Management - How to Post a Screenshot of Windows 10 General Tips Tutorials
Make sure to widen the columns so we can see all the data in the columns.

Now we are going to try to get back into Windows 10. If the bcdboot command messes up the Windows 7 boot files, we can use the screenshot of disk management and Kyhi's recovery tools to rebuild a boot partition, but the bcdboot command, by itself, should not break booting unless there is an underlying corruption somewhere else.

"Run as Administrator" Command Prompt again:
bcdboot E:\Windows

This time leave off /addlast /d. This should put Windows 10 at the top of the Windows boot menu and set it as the default OS to load. Reboot the computer, let it go into Windows 10. Also, change the drive letter if you have to to point to Windows 10.

If you get into Windows 10, open a command prompt (admin) and run the following command to turn off hibernation and delete the hiberfil.sys file, which can cause problems when dual booting with Windows 7:
powercfg -h off

Report back and let us know how all that went. Then we will work on rebuilding your boot partition on your primary drive.

A few quick things as I am at work right now.

1) I still have access to my Windows 10 HDD in Windows 7's file explorer so I've been backing my stuff up.

2) Since Windows 7 is running just fine can I simply reinstall Windows 10 and have it solve the dual boot issue? By that I mean finally giving me the option to select an OS on boot? Because I know you're supposed to install the older OS first.

If it will I might just go ahead and do that unless this is fixable. I already had backed up my files about a month ago so it's only a matter of getting a few more. I don't mind to reinstall if that's a guaranteed fix.

If you are looking for the easiest solution...
Unplug all the drives except the one you want Windows 10 on. Reinstall Windows 10. Open the Command Prompt (Admin) and turn off hibernation, which will also disable fast booting:
powercfg -h off

Shutdown by holding the shift key when you click shutdown from the power icon on the start menu (this will ensure you are doing a complete shutdown, not a "fast starup" shutdown).

Reconnect your other drives. Reboot into Windows 10.

Open Command Prompt (Admin) again. Run the BCDBOOT command again to add the Windows 7 to the boot menu:
bcdboot E:\Windows /d /addlast

Adjust the E: to match the drive letter assigned to the Windows 7.

That's the easy way to take care of all the problems.

This seems no different than what I did in the first place. Will this be different because technically when I reinstall Windows 10 that Windows 7 was first going with the whole install old first and new last?

RECONBunny said:

This seems no different than what I did in the first place. Will this be different because technically when I reinstall Windows 10 that Windows 7 was first going with the whole install old first and new last?

The clean install will fix whatever file system corruption caused the problem. Disabling hibernation (and, therefore fast startup) before booting into Windows 7 will help ensure it continues to dual boot smoothly.

AddRAM said:

In my opinion, keeping 7 and 10 totally separate is the best way to do it, and when you want to boot into the other drive, just use the one time boot menu to choose the other OS.

Set your preferred OS as default in the Bios and it will always boot to that OS.

Oh I know but I like having that menu pop up and ask me what OS to select.