Windows (Boot Camp) won't boot from HDD in optical drive bay

You have not begun to given enough information to correctly diagnose your problem. This answer this therefore is based on many assumptions.

I know Windows 10 can be installed using the EFI boot method on the newer Mac computers. I do not know if this includes your model. Although, if your Mac does support booting Windows via EFI, then, I believe you should not of had any problems booting.

I have read from some Apple web sites where Windows can be installed on a second hard disk by temporally replacing the first disk with the second disk. If such instances, Apple referred to the computer as a Mac Pro. Your computer is a MacBook Pro. Mac Pro computers are designed to support a second internal HDD or SSD. Your computer probably was not.

For the rest of this answer, I will assume you tried to install Windows to boot using the BIOS method. It would seem that if you are intent on keeping OS X and Windows on separate internal drives, then the answer would be to install Windows on the first drive and OS X on the second drive.

Another option, if the BIOS boot method is to be employed, would be to boot Windows from the first drive while keeping most of the Window on the second drive. To do this, you would have to create a small NTFS formatted "System Reserved" partition on the first drive. This partition can be as small as 100 MB, but 500 MB is the recommended size. This partition would also have to be marked as the active. This would also mean the first drive would be using a hybrid EFI/MBR partitioning scheme. The second drive would use the legacy MBR partition scheme. Once the "System Reserved" partition is in place on the first drive, you can simply boot the Windows installer from the USB flash drive and install Windows 10 to the second drive. The installer will automatically know which files need to be placed in the "System Reserved" partition.

If you answer questions about setting up a "System Reserved" partition, let me know.