Really good points by
@nRiTeCh. It is because of these reasons that I too like to install different OS on different hard drives.
I am mostly worried about having problems with MBR because of problem in one drive or other. If that happens, I cannot boot any OS, and have to repair the MBR first. That in itself is a thing to learn, and there can be different scenarios for that.. having two Windows, or having Windows and Linux on a drive.. all might have a different way of being solved. I will first have to search for a solution, and again, if it does not solve, then I am stuck without a bootable OS... and this makes the task of performing backup difficult too.. as you will then have to put in a live Linux CD, and perform backup on an external drive. Whereas, if one OS is booting from one hard drive, I can just take backup of other hard drive, without going through much process.
Also, there are other problems, as
@nRiTeCh suggested.
So, I find it much better to have different OS on different hard drives.
Also, I mostly keep one OS as my main, and that is Windows. The other I keep changing. Sometimes I install different version of Windows, as an experiment, or a base to install and check software. Sometimes I try out different Linux distros.
This frequent change of OS is quite convenient if I have a different hard drive to install on. I can simply format the hard drive, and start fresh. I can perform all sorts of experiments on it, without risking, or having any effect on the other hard drive, which contains my main OS for daily use.
I do not have to worry about corrupted MBR, etc.
Also, it's quite easy to boot from the required hard drive. All you have to do is change the boot order, and that's it.