My friend has a computer with a motherboard exactly like mine. So, I connected a third hard-drive, sdc, on which I installed Debian together with another minimal installation to contain and manage grub-pc. The latter's primary stage was installed to hdc's master boot record. The hard drive was formatted as a GPT drive. grub-pc installed successfully because I could boot Debian on my friend's computer.

The problem is I want Windows' bootloader to be called by the bios. The reason is because my friend wants to keep his Windows installation as intact as possible and because grub-pc may fail rendering his machine unbootable even to use Windows.

I read that one can use boot.ini by adding another line to it like the following:
C:\primary-stage.bin="GNU Linux". Primary-stage.bin is a copy of the boot sector of hdc.

My question is this: how would primary-stage.bin know which hard drive's sectors to load and execute when the disk containing these sectors is configured as the second hard drive?

ADDED LATER:
I found that editing C:\boot.ini by adding the line:
doesn't work as I correctly suspected. The reason for this failure may be that the boot drive as seen by the BIOS was not the primary boot device.

The file C:\grub_bootsector.bin is a copy of the second hard-drive boot sector.

You might check the site below which is about as detailed as I've seen to do what you want. You need to scroll down the page a little to get to the Linux part. This is for xp as vista, 7 and 8 no longer have a boot.ini file and use a different bootloader.

Any use (that I've seen) of adding the Linux boot sector record like that pre-dates gpt disks. I've never tried it using gpt.
It works (on MBR disks) if you install grub to the partition then copy the first sector of the partition - not the MBR.

Doesn't matter if it's the first disk or not, so long as the disk isn't moved after the setup command is run - that saves the enumerated disk id as well as partition number.