As mook765 noted in a comment, this cannot be done with GRUB. The trouble is that most EFI-mode boot loaders and boot managers, including the EFI-mode version of GRUB, can launch OS kernels in EFI mode and/or chainload to other EFI programs; but they cannot launch BIOS-mode boot loaders, which is what you need to boot your existing BIOS-mode version of Windows. I know of three exceptions to this rule:

• The firmware's own boot manager -- Most recent EFIs with BIOS-mode support (provided by a Compatibility Support Module, or CSM; also referred to as legacy-mode boot support) provide a built-in boot manager that enables you to switch between native EFI-mode and BIOS/CSM/legacy-mode booting. You typically enter this boot manager by hitting Esc, Enter, or a function key as the computer is powering on. This can be awkward, and in some cases it's not supported at all.


• rEFInd -- My rEFInd boot manager supports switching from EFI-mode to BIOS-mode booting. On Macs, this ability is enabled by default. On UEFI-based PCs, though, you must edit refind.conf: Uncomment the scanfor line and ensure that hdbios is one of the options. Since you're trying to boot Windows from a second hard disk, you might need to uncomment the uefi_deep_legacy_scan option, too -- but I recommend doing this only if doing it without that option doesn't produce an option that boots Windows.


• rEFIt -- This is rEFInd's predecessor project. It supports BIOS/CSM/legacy-mode booting on Macs only. If your computer is a UEFI-based PC, it won't do the job. If you've got a Mac, you could use rEFIt for it; but as rEFIt has been abandonware since 2010, rEFInd (which I'm still developing) is probably the better choice.

Note that all of these options require that the firmware provide a CSM, and that it be enabled. AFAIK, all Macs (with their latest firmware) and most UEFI-based PCs provide a CSM, but a few UEFI-based PCs lack this feature. If you've got such a computer, your only choice is to convert the Windows installation to boot in EFI mode. See this page for instructions on how to do this. You might want to go this route even if your computer provides a CSM, since a CSM slightly slows and complicates the boot process. OTOH, it's likely to be easier to set up rEFInd or use the built-in boot manager. You may need to enable the CSM in your firmware setup utility. Details of how to do this vary from one computer to another. Typically, there's an option called "boot mode" that you'd set to "UEFI or legacy," or some variant on these option names.