I had something similar with ASUS once.
Their BIOS on two different laptop models stopped you using encryption while in IDE mode, back when you still used IDE mode to install Windows XP.
Worked fine if you used XP in IDE mode. Worked fine in AHCI mode. But if you tried to change the disk format in IDE mode, it threw a fit and refused to boot.
Turned out that any non-NTFS partitioning, weird NTFS partitioning, or encryption would render it unbootable. We found out when TrueCrypting a bunch of the laptops.
Turns out that their BIOS was explicitly checking for a zero at a certain hexadecimal offset into the drive which was ONLY true if you used NTFS partitions that covered the entire drive and never encrypted. Which, obviously, their pre-fab images did.
We threatened to send a whole bunch of purchased laptops back because of it, and as soon as money was involved, our suppliers, ASUS and AMI suddenly sprung into action and patched the atrocious bug that they'd introduced by the same kind of "Oh, nobody uses anything but the default Windows install" thinking.
We ended up getting a replacement firmware that worked, even if it was marked as beta.