I have Windows 8.1 on one HD. I installed Windows 10 on another HD. I have a 3rd HD where I store music, photos, movies, and other stuff. When I finished installing Windows 10, I could access all my files on the 3rd HD. Then I rebooted into Windows 8.1. There I couldn't access any of the files on the 3rd HD. The security descriptors had changed. So I changed them back and was able to access the files again. Then I rebooted into Windows 10. I couldn't access the files on the 3rd HD. Is there a way to allow me to access the 3rd HD files from either OS without having to change the security settings over and over?
My original setup was 8.1 and 9926 on two partitions on the same drive - then in second-stage upgrade testing, I upgraded the 8.1 partition to 9926. (THe result was two partition with 9926 - the old clean-install partition, and the upgraded ex-8.1 partition. Due to compatibility issues with three games (all from Sony Online Entertainment, and all using SOE Launchpad), I created a small partition using an 80 GB SATA HDD just for 8.1. Result, I have the original 8.1 partition (now upgraded to 9926), the original 9926 clean-install (second partition of the 1 TB main HDD) and the 80 GB second HDD (with 8.1 and the SOE games by themselves). Once I get past the UAC prompt, I can access any file, on any of the three partitions. (Because the three games are the ONLY incompatible software I have, that is why the partition can be that little - thus no need for Office 2013 on the 8.x partition; I DO have VLC, but that is to play media files on the other two partitions - it remains my default media player across all of Windows, as it has been since 7. (Yes - that includes Windows Server.) In other words, nothing else has broke. I now have three partitions (on two HDDs) and don't have that issue.
Not really sure of your question... I have a total of 7 drive partitions on 4 separate drives spanning 2.5TBs... I have an 8.1 partition and a 10 partition on my boot drive, and I can boot back & forth and access all drives from either installation without a problem. Are you using the the Win8.1/10 boot menus to boot, or are you simply selecting your boot drive from your bios/UEFI...? If the latter, that may be the problem--you should be booting from the OS boot menus. Also, make sure that the third hard drive letter is the same letter under both OSes--this is for convenient pre-installed application access under each OS (although that should not keep either OS from seeing the drive.)
Thanks for your suggestions WaltC. I am using the Windows boot menus. When I want to switch, I go to System/Advanced System Settings/Startup and Recovery and change the default Operating System. And yes, my third HD has the same drive letters in both OS. No, I'm pretty sure it has to do with security settings and ownership rights.
PROBLEM SOLVED: In the $RECYCLE.BIN folder there were multiple files with a name such as S-1-5-21-2287498343-1216553579-3284852717-1001. I deleted all but the 2 with the most recent dates. Now I don't have the problem any more. I think each time I installed a newer build of Windows 10 another one of those S-1 files was created in the $RECYCLE.BIN folder and they were the cause of my problem.