Truecrypt's website claims the following: Which implies that a hidden OS partition should be indistinguishable from a standard encrypted OS partition; i.e. there should be no "flags" telling the bootloader that it is booting into a hidden OS rather than a normal encrypted OS. Futhermore, the hidden OS partition is supposedly a clone of the system partition, which, if that's indeed the case, further implies that the bootloader not only has no indication that it is booting a hidden system partition, but that it actually thinks it is booting the standard system partition. Now, if this is truly the case, the following should work: 1. Encrypt the system partition using the standard method 2. Create a partition directly after the system partition, encrypt it with Truecrypt. 3. Create a hidden volume in the partition identical in size to the system partition. 4. Mount the new hidden volume and manually clone the system drive to it. 5. Reboot, and enter the hidden volume password. This process should create a hidden operating system which should successfully boot when the hidden volume password is entered. The problem, as you've no doubt guessed, is that it doesn't, and instead gives a rather puzzling "incorrect password" error, even though the volume mounts correctly when the same password is entered from Windows. This is major, because it means that the claims the development team made are false; there must be something telling the Truecrypt bootloader that a hidden OS is present or not, and if it doesn't "expect" one to be present, it doesn't even attempt to boot it. If, as the developers claimed, it simply attempted to mount the hidden partition, and if successful attempted to boot from it, then there is no reason why the hidden system wouldn't boot after following the above instructions. Even if the cloning process somehow corrupted a system file or was not done correctly, it should attempt to boot and give a BSOD. Heck, even if the hidden partition was empty it should still mount successfully and subsequently hang on an "operating system not found" error when it tries to boot and finds no MBR. Yet that is not what happens, instead I get an "incorrect password" error when I enter the correct password for the hidden partition. This means the bootloader isn't even trying to mount that partition and is simply rejecting the password after trying to mount the main system drive. Somehow, the bootloader knows that I haven't "officially" created a hidden OS, and so doesn't bother attempting to mount the hidden partition, even with the correct password. I find this extremely worrying, and was wondering if anyone had any thoughts or could help shed some light on what's actually going on, as opposed to what the documentation claims is happening. Feel free to replicate the results I got yourself using the instructions I've listed above. I'm going to look into things some more, but I'm not liking this at all.