But when I installed it, I created a single primary partition (on a MBR drive) and installed windows 10 into it so I could end up with a single partition. I installed windows 10 just to get it while it was free, I prefer windows 7 so I don't use it at this time.

I hardly ever have to boot into XP to access them and someday I'll convert them all to current versions of the program. The XP installation is from my old computer I used a couple years ago, I brought it along because it has many years of company data and a few older versions of my bookkeeping software that predate windows 7 and 10 and therefore do not support it. This system I'm on right now in my office has 3 versions of windows: XP, Win7 64bit, and 10 64bit. I don't need to use 'bitlocker' and that's the only reason I know of that requires a 'system reserved' partition on a MBR drive, I would rather have all the boot files in the single operating system partition. I also, along with Brian, prefer to have operating systems installed into a single primary partition because it makes it easier to image everything and keeps them self contained. I really only want operating systems in primary partitions. When multibooting you could install operating systems (at least some versions of windows) into logical drives, personally I would not do that for my production systems although I have done it many times in the past in test systems. However, one primary partition slot can be used to create an extended partition containing as many logical drives as available drive letters allow. When I said 4 partitions for a MBR drive, I should have clarified that. Very good to see you Brian, hope everything is well and I hope you keep in touch. Desktop motherboards allow so many hard drives these days I just use a decent sized SSD for windows and multiple 2TB drives for everything else. I still dislike UEFI systems and have no need for more than 4 partitions per drive or single drives over 2TB. I also recall taking the same Windows 7 UEFI image and easily making it boot as a regular MBR partition by adding the 'Boot' folder from another installation to the single new MBR partition and then replacing one file, if I recall correctly it was as simple as replacing 'bootmgr.efi' with the non-uefi version 'bootmgr' (no extention). I can't remember if it booted right away or if I had to fix the partition signature for it to boot but I recall it being pretty easy to make it boot. I never actually did what I suggested with Windows 10 but I have done it with Windows 7 64bit installed as UEFI. One of the partitions is the 'Microsoft reserved partition' (MRP) that is empty and as far as I know it is still unused.

Make windows partition bootable