For some reason the upgrade process is detecting your other drive as the boot drive but it can't find the OS there that's why the upgrade can't go through.
It's a common problem with this upgrade that's why I don't personally recommend dual booting with Windows 10 Preview with any other OS including Linux even on separate drives. You will always run into boot issues. Personally before doing any upgrade, I pull out my other drives to avoid that issue. It's easy for me because my hard drives can be accessed from outside the case.
But my main OS (in M.2 SSD) is in GPT disk and my Insider Preview version is in MBR disk. The 2 don't communicate so I don't really need to pull out the M.2 SSD.
Oh my.. now that is .. well.. NICE!!
Last edited by Nikecow; 31 Jan 2016 at 16:31.
I've just moved to 14251... OK
Nice case there...