You don't say what type SSD, specifically if your SSD is NVME, and where you have these installed. The defaults for your new MB might be UEFI which uses a different kind of boot sector than your old hard drive/MB. Whatever all that, you do not need to boot on your old hard drive to install W10 on your new ssd. You can clone your old hard drive using bootable software installed on a stick or DVD. Better option would be to download the latest W10 iso file to create a bootable stick or DVD and do a fresh install. You already have all your personal files backed up on your old hard drive--take it out of your system for now. After you get W10 installed and updated on your new ssd, you can reinstall your old hard drive. I expect your HD has a few years on it--not that it is wore out but that there are better options for cheap storage. I would replace it myself. At any rate, copy all personal files--docs, photos, videos, etc... to somewhere else--a stick, your c drive, or in the cloud as you like and then I would reformat the old HDD to clean it up and be quick about it if I had to use it.
You might need to purchase a license for W10--depends on how you acquired your original. I also assumed you have flashed the latest appropriate bios before worrying with your OS.
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