Welcome to the community, haohao!
I'd recommend you to make sure that the booting order is properly configured in the BIOS settings. Did you unplug the primary Windows 7 booting HDD when you were installing Win 10 on the secondary drive?
I'd also suggest you take a look at the connections inside your rig and make sure everything is mounted properly. Another thing you should look is the SATA controller and chipset driver updates from your motherboard manufacturer's website.
If you haven't done that already, I guess you might also give
the BIOS reset a try. It would get all your BIOS settings back to factory defaults.
Hope this helps.
SuperSoph_WD