Win10 update, now can't boot...

[deustroop]

Well I have never thought that having two OSs on one drive was recommended in any event though so many have just that modus operandi. The installation process of updates and upgrades can easily install incorrectly mixing up what goes where when both OSs are a windows product. I have a tri boot system but each OS is on a separate drive. Even then I disengage two of the drives whenever windows installs an upgrade that requires a reboot, before I allow the reboot. The precaution is also important when doing a fresh install: only allow the installer access to one drive for the install, to reduce the chance of error. This is now a truism of windows operations.

 

[Ketchup]

You have discovered why it is not recommended to run two boot drives with the same OS. If you need a second drive running Windows 10, I would recommend using a virtual machine. Much easier to set up. For now, the best recommendation I have seen is for you to backup your data, run a clean install with just the new drive connected, and format the other drive (or otherwise remove it from being a boot source).

 

[bruceb]

Not too sure about dual boot issues with Windows 10 .. But if you did want it I would do it this way:
New hard drive with 1 partition for each OS
Make sure all of it is NTFS
Disable Secure Boot
Install the Oldest OS version first
Then Install the Win 10 version
If needed add a Boot Manager to help with selecting the OS on boot up.