UPDATE: Thank you for your responses and helpful ideas! I was able to fix the Mac-related issues by upgrading to Sierra - not High Sierra - which can support both Logic Pro 9 and Logic Pro X 10.4 concurrently. This is not stated at all on Apple's site. At any rate, I was able to now only need a Mac partition and a Windows partition. I have had some other catastrophes to deal with in the meantime, so I have not had the time to fix the Windows side of things yet. I will report back when I do!
I'm going to try to run down this issue as succinctly as possible, but bear with me; it's convoluted.
Last year my main 1TB HDD in my mid-2010 Mac Pro showed signs of failure. I proactively bought a new 2TB SSHD, made an image of my El Capitan Mac partition, and did the same with my Boot Camp Windows 7 partition. I formatted and partitioned the new drive appropriately and restored those images onto the new drive. MacOS worked great, but the Bootcamp partition refused to boot, no matter what I tried. However, I could still boot into Windows from the old drive just fine, so I left it in the machine. I ended up using a Windows program to repartition that drive into its own Windows drive, using the entire 1TB. I left the Bootcamp partition intact on my new drive in case I figured out how to fix it.
Another weird quirk of this configuration is that when I am booted into OS X, I can't see my Windows drive at all. It shows up in Disk Utility but is completely unreadable. However, when I am in Windows, I can see all my Mac drives as normal.
Fast forward to yesterday. I am a music producer and game composer, and the project that I'm working on would really benefit from me upgrading to the latest Logic Pro X from my current Logic Pro 9. Logic Pro X 10.4 requires High Sierra. Logic Pro 9 does not work in High Sierra. I need both for backwards compatibility reasons; Logic Pro X ditched the 32-bit bridge, and I still have many projects in progress that use 32-bit plugins. I need both. So I just finally ditched that Bootcamp partition that didn't work, cloned my MacOS partition, booted into the cloned partition, upgraded to High Sierra there, got the new Logic working.......
And now I can't boot into Windows. It's on a separate hard drive! I thought this would circumvent the 4 partition limit.
To clarify and summarize this issue, my main hard drive now contains two partitions; one for El Capitan, and one for High Sierra. I have a separate HDD that contains Windows 7 which was originally set up via Boot Camp Assistant, but went through the aforementioned changes. I need to be able to boot my Mac Pro into any of these three OS'es at any given time.
What in the world can I do? Can I finally have some peace and harmony in my computing life for once? Has anyone encountered anything quite like this? I'm pretty frustrated.