It isn't really a BIOS issue. Dual and multi-booting can be accomplished in a number of ways. Read the tutorial at the following website to learn how you can do it. You should follow the Windows installed first method. Use a Grub loader.
That didn't works. I'm on Windows 10 Pro and using to install Linux Server on 2.HDD, will install the Grub Loader, just didn't give the option by booting to chose the OS! Instead it also crashed my Windows 10 Installation totally and I've to do a clean new install of it as well.
Any other way for to get more than one OS working on an HP Compaq Microtower dc5800?
Sorry if I could not spend weeks and month for to "maybe" get something to work, if ever.
The simple fact is, after following your advice for to install the Linux Server on the second HDD and using the Grub Loader, my original and activated Windows 10 Pro was simply spoiled by the HP Compaq Microtower dc5800 computer, so I need to spend quite some time for to do a clean new installation of Windows 10 Pro and all apps again, because a Repair wasn't possible!
That's not the first time I get such crazy problem regarding incompatibility with some HP Hardware and/or Driver and never got a working help from there!
Sorry to tell that, it's simply the truth!
In this case of that computer, it's just because of an idiotic technical design for not to be able to use any of the SATA HDD's for to boot on demand to an OS installed on it using a simple BIOS Function, because the BIOS Function didn't exist! 99% or even more of all Computer having that function!