First at all: I didn't talked about the Bootloader instead I talked about the Boot Manager, which is quite different!
Secondly: if using several HDD's with different OS's (installed on it's own HDD) and just ONE Bootloader on the Main HDD, while that Bootloader is configured for also to boot the OS's on that other HDD('s), what's happens if that Main HDD is failing and unable to start up any OS?
Simple.
In the case that you describe, you can still change the boot drive in the BIOS, and boot from any other HDD with its own OS, provided each one was installed with just that HDD connected, and/or the HDD each OS was installed to was first in the boot sequence of the BIOS at time of install, so whatever MBR/bootloader which the installed OS uses was installed to that HDD, and nowhere else.
In the later case may it's a good idea to have min. the Main HDD running as RAID 1.
But use our way you don't need that, just hit the required F-Key while boot the computer, choose the OS/HDD you like to start and: VIOLA that just works.
Yes, but you're still using the bootloader on the HDD you change to, to boot.
If you have the MS boot manager configured with EasyBCD on the main HDD controlling the boot, then it wont take away your ability to still change boot drives in the BIOS should your main HDD fail. It only enhances your multiboot.
Don't get me wrong, I didn't tell with any word that EasyBCD isn't good, which it isn't! But we would never use it because to be 100% sure that we are able to boot up our WS's even if the Main HDD fail.
And again: I talk about a Boot Manager.
First of all, when one talks about "Boot Manager", one is usually referring to the boot manager file which handles the boot process of the OS, unless that person is incorrectly using that term (as you seem to be...). The "BIOS Boot Manager", as you called it, is not a boot manager. It is only a program in the BIOS which chainloads the MBR of the HDD set as the boot drive in the BIOS. The MBR of that HDD with then chainload the PBR (partition boot record) of the "active" partition on that HDD, which then loads the boot manager used to load the OS(es), which in the case of Vista/7, is the file called "bootmgr" stored in the root of the "active" "system" partition (according to Disk Managemt), which uses another file called "BCD" for storing the boot entries in your boot menu. Also, using EasyBCD to add entries to your main HDD's BCD after the install of all OSes (like previously stated) will NOT effect your ability to still stand-alone boot each OS, if they're each installed to their own HDD. So even if your main HDD fails, you will still be able to change the boot drive in the BIOS, and load the OS you want.