The MIFD/DMI data is in flash and not on the HD. The MBR is the first 512byte sector on the HD and is full of bootloader code and the partition table. Needing to wipe the MBR occasionally occurs if you've damaged the MBR in some way or if you've loaded a replacement bootloader and now no longer wish to use it.

The reason why you're desktop attached laptop drive didn't work when back in the laptop is that Windows doesn't appreciate all of it's hardware changing at the same time. The HAL it uses is fixed as well so if it was using the ACPI one on a non ACPI machine then it'd hang.