System/Boot files on different drives

New to these forums and hoping someone can help with my issue.

I have a homebuilt machine that had Win7 ultimate running on a 150GB HDD. I cloned it some time ago to a 500 GB HDD. Then I changed to Win10 Pro (first upgraded to Win8 and then immediately to Win10) and all seemed to be ok. I am wanting to upgrade to an SSD but have found this problem:

According to Disk Management, my Windows system and boot files are on different drives.

C: Boot, Page File, Active, Crash Dump, Primary Partition (this is the 500GB)
Z: System, Active, Primary Partition (this is the old 150GB)

I can't seem to post an image (but attached pix) so if there is something else that will help, please advise.

I have gone through numerous attempts to fix this by rebuilding the BCD, cleaning up old install files and now when I boot up, I get a window asking which OS to boot (10 or 7). I want to get rid of the 150GB drive completely, but when I disconnect it I get the missing winload.exe blue screen so something isn't right with the Win10 Pro install.

I ask the community for help so I can move forward with an SSD upgrade.

Welcome to the forum. You dont say if windows is updated to the latest win10. download the lates up to date and put it on dvd or usb. remove z and i put c on disk 0 controller the one z is on. boot the dvd and do a upgrade once working you can put other disk back never install with more than one disk as it can put files in the wrong place.

If you want to use a ssd its best to install that and do a clean install with that as the only disk it will then be optimized for ssd and by far the best option you wont need a key as activation is stored on MS server

Be aware that Samuria's plan has a flaw in it. You can't upgrade the existing Windows 10 to a higher build by booting from a USB flash drive or DVD. You must run setup.exe from within Windows 10. If your current C: drive partition does not have the boot files included on that partition, when you physically remove Z: drive you won't be able to boot your computer into the existing Windows 10, and thus, you won't be able to upgrade it to the latest version contained on a USB flash drive or DVD.

I would start by attempt to establish/fix the boot files on your C: drive partition by opening a command prompt (admin) or powershell (admin) and running:

bcdboot C:\Windows /s C: /f ALL

If that completes successfully, then try booting the computer with the Z: drive unplugged. And I do agree with the recommendation to moving the drive containing the C: partition to lowest numbered SATA port where the Z: drive is currently plugged in.

If that doesn't work, then you will want to boot from a Windows 10 installation USB flash drive or DVD and run the bootsect command. I would have to reboot my computer into recovery to even come close to getting an example correct of how that works.