<<So, GRUB is installed in your system's primary drive (which is where it "wants" to install itself by default).>>
<<At start-up time, does GRUB provide you with an option to book Windows or Ubuntu?>>
Yes it does.
<<If you select Windows, does the system correctly boot into Windows?>>
<<And does your Windows system still work, letting you run Windows programs, seeing the correct drives and disk space, etc.?>>
<<Finally, if you select Ubuntu at start-up time, what happens? >>
It boots up (fast!) and runs fine.
So you see, everything is working as is, so why I am I doing this? LOL. Well, the main reason was to leave the Windows drive untouched, so it wouldn't depend on having the Linux drive in the computer. Second, I was planning to install another Linux distro in another partition (on the slave drive) and use the same technique for it, using the Windows boot menu to select between them all. I was also thinking that having Ubuntu's boot menu there on hdb (but not being used), would allow me to boot from either drive as a stand-alone on the master if I ever want to (but I made the mistake of not mapping the first partition as /boot during the install).