A system and method for Flex Over or On Suspension (FOS) fault detection under a write gate for magnetic disk drives may employ a flex over suspension fault comparator that compares a predetermined reference threshold to a positive delta of a disk drive write head data stream and outputs a positive or negative FOS fault indication. A max/min buffer detects polarity of a disk drive read/write head and applies a positive buffered delta to the fault comparator. A transition-free window detector triggers the fault comparator to output the fault indication when a transition-free window of zero-value data bits of a predetermined length is detected from the disk drive head, and the transition-free window detector provides a fault validation signal to validate output of the fault comparator when the transition-free window of zero-value data bits of the predetermined length is detected from the disk drive head.