Accessing old drive from new drive OS - security problems, corruption?

I recently got a new SSD to compliment my old HDD. I decided to do a fresh install of Windows 10 on the SSD while leaving the old HDD operating (upgraded Win10) system intact, with the idea that I could boot into the old OS to use any programs that I couldn't easily reinstall or that I didn't want to take up space on the new drive because I rarely use them. I would also use the HDD to store all of the data, e.g. pictures, documents, etc. (at this point all of my data is still on it anyway).

I installed windows with the old drive unplugged, then plugged it back in once the new one was set up. At first it went smoothly; I was able to boot into either drive from the BIOS. When I first tried to access files from the old drive's user (F:\User\<username>) it prompted me to take control of the files, which I unwittingly agreed to.

This still seemed to be working well: I was able to see all my old stuff and access some files from the new OS. But then I switched back to the old OS to export my outlook rules and programs started breaking, and I now didn't have access to various files (including some basic programs and OS services) from my original account. From there things are a little hazy: I started switching back and forth between OSs, messing with the security tab, e.g. trying to allow access by my Microsoft account (<address>@gmail.com) since it is the address that controls both logins. I kept running into problems though: either the subfolders/items are not under my control or, when they are, that the contents are "corrupted and unreadable." Finally I try to run a disk check on the old drive and on the next boot I get immediate blue screen with stop code NTFS_FILE_SYSTEM that kept restarting and dying again.

I switched back to new OS and took the "take control" prompt one more time. The file structure is still intact. At this point I can navigate all the folders and see all the files, but I can't open or copy documents because "You require permission from the computer's administrator to make changes to this file." On the old drive it now boots up to the repair options state.

Is there any way to save my old OS in its entirety? Is there any way to save my data (docs, pictures, etc.) even if the OS is unusable? I'm happy to clarify anything that I can still reproduce now. It may be somewhat difficult to elaborate on the things I did to get to this point, but I will try if you ask.