An error in the Toshiba ACPI BIOS 1.6 causes the BIOS to only examine the first slot in the Master Boot Record (MBR) table for an active partition, which prevents the system from booting even though the MBR is not malformed.  NOTE: it has been debated as to whether or not this issue poses a security vulnerability, since administrative privileges would be required, and other DoS attacks are possible with such privileges.