How do i add another windows boot option on the same hard drive?

Show us an image of your disk management page...

Berton said:

It probably requires a boot loader to run multiple versions of an OS. An example would be GRUB, frequently used with LINUX. I don't multi-boot as I have sufficient computers on hand, mostly rehabbed rather than throw out, and am now running Win10 Home, Win10 IP Pro, Linux Mint 18 and Win7 Home Premium computers on a single keyboard, monitor and mouse through a KVM switch.

If I were to run multiple OSes on a single computer I'd do it with either a separate partition for each on the only HDD or multiple HDDs with one OS on each. Or I'd run virtual machines for the 2nd or 3rd OS.

I am betting reason is because OP has legacy bios installation's and has run out of primary partitions. You do not need a boot manager like grub.

There is an easy way round it without mangling existing partitions which is to install it in a virtual hard drive. OP need to post image of disk partitions for use fo advise best solution.

NavyLCDR said:

Or put two or more of the operating systems in logical drives inside an extended partition. Only the partition the computer boots from needs to be a primary partition, the partition(s) holding the operating system(s) do not.

That will work as well but I prefer vhds as easy to create and delete and only impact the sytem reserved partition. With expandable vhd, you do not need to reserve any partition space as well. They just use what they need up to maximum size.

Although Windows Setup may be used to add a new multibooting Windows OS to a system that already dual boots Windows, I don't trust the new Windows installation not to hijack the active system partition when that may have been created by an earlier numbered version of Windows, especially if I mistake the location of the setup.exe file.

I'd rather add the new partition myself using Disk Manager, which usually handles extended and logical partitions well, or Diskpart, and use DISM* to apply the new Windows installation. See Apply Windows Image using DISM Instead of Clean Install Installation Upgrade Tutorials

VHD native booting Windows 10 installations have the disadvantage of being unable to update to a newer build, which come surprisingly frequently, even for feature build updates.

Typically for Windows 10 Insider clean installations (to which I add very little additional software, and expect to update with a new build within a week or two) I allow about 35 GB for the partition.

If the new partition is assigned the letter N:, then to add the new BCD bootmenu entry to the BCD on the active system volume, use the command Bcdboot N:\windows

*I'd sooner use Imagex than DISM, simply because the syntax is much briefer, so less chance of typos, and the wimlib-imagex.exe (also found in ESDtoISO or UUPtoISO bin subfolder) is even briefer.