Title pretty much explains it. Have Windows 10 Preview on another hard drive but it's not coming up at all on msconfig so I can choose between the two operating systems in win boot manager.
Having the OSes boot independently via the BIOS is the best way to configure it so that neither is dependent upon the other to boot.
However if you don't wish to boot your choice via the BIOS then you can install EasyBCD (click Download - no Name or Email required) to the primary OS (on the HDD which has the System Active partition) to add the other OS to a Windows Boot menu.
If you have any questions please post back a screenshot of Disk Management - Post a Screen Capture Image
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