HDD read head signal
In a word, no.

First of all, GMR heads need to be biased to a particular operating point in order to see the GMR effect at all.

Secondly, the heads are physically designed to be most sensitive to a magnetic field imposed across a tiny gap that rides very close to the surface of the disk. On a modern HDD, that gap is extremely narrow &mdash; it's actually smaller than the tiniest transistors in a modern VLSI chip! To whatever extent is possible, the head is designed to be insensitive to all other imposed magnetic fields.