Assuming there are two OS installed on a hard drive, is there any way to make one OS boot by default and the other only boot if a key is pressed during start up? I was talking with a few friends about an episode of NCIS where something similar to this happened, we began wondering if it was possible to make an OS unbootable unless the booter knows how to make it boot.

The idea is to have XP boot every time the computer is turned on unless the operator presses F2 or backspace etc. and then Ubuntu or similar OS would be used.

I apologize if this is the wrong forum, but for such an odd question i thought it best fit here.

If you are a competent asm coder, it is trivial to write your own code to put in the MBR to do whatever you like at boot time, including checking for a specific blind keystroke with a short timeout to boot a very well hidden OS. (I've coded and used that several times for various reasons).

Hiding an OS may be a harder problem, depending on the level of expertise of the person you want to fool. Hiding it from an ordinary user who just sees what the booted (non hidden) OS shows him, is pretty trivial. It is vastly harder to hide it from an expert who expects something hidden to be there and comes armed with his own bootable CD (so he can trust what his own OS tells him about your hard drive).

However well or poorly you have the OS hidden, the well disguised secret boot method will probably be a minor part of the total effort.

Grub (the common Linux boot manager) can be easily customized for what it looks like while booting, how long it waits for keystrokes and what it boots by default. So to fool ordinary Windows users, you might not need to hide the Linux install at all nor do any of your own programming. You might just configure GRUB to be fairly silent and to default to booting Windows after a fairly short timeout.

I watch a lot of similar crime solver fiction, but not that particular series. The "science" mentioned and used in those shows is almost always total nonsense. But extrapolating from what you posted, maybe this time they were moderately close to the truth.

It is quite trivial to make an OS easily bootable to someone who knows the secret while it could be totally unbootable even by an expert who knows it is there and invisible to a non expert who doesn't know how to look for it.

Making it invisible to an expert would be borderline impossible. But making it useless to even an expert who could easily find it probably just needs some decent encryption plus attention to a whole bunch of complicating details.

When I was still using multiple OSs I used a three drive system, one for data(stays in machine at all times), and two for the OSs(removable). Drive slides (removable drawers) are great for this type of thing. Put each OS in a separate drive and slide (both are relatively cheap). When you want to change OS, shut down machine, turn key, pull drawer, put in other drawer, turn key, and boot. This method has the advantage of completely isolating each install, while still allowing you access to your data. At one point I had 7 OSs set up this way. It also allows you to secure the unused drive.

Unless there is something I didn't get here, this is a piece of cake if you use the GRUB bootloader. You only need to make the bootloader hidden; if you want to boot a different OS, you press ESC at boot time and you get to pick OSes from a menu.

Thank you all. I understand that completely hiding the OS would be way over my head but I might try my hand at configuring GRUB. A few hours of Googling should give me some idea of what I'm getting into.

Edit: For anyone wondering, the episode of NCIS involved someone hiding a missile guidance system in an old(very old) laptop case. After looking on Wikipedia i think it is called Blowback Season4 Episode 14.

If that's the one I think it is, they showed it here just recently, Channel 10.
At the moment they seem to be showing double episodes, but each one from a different series, a bit confusing sometimes as the cast has changed (new NCIS Director)

I think if you have Windows as the second OS you can edit the settings from the boot.ini file. You can remove the name of an OS from the boot.ini file and which will not be displayed next time.

[ I am sorry I don't know very much about Linux ,I am in the learning process only]