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Hard Drive Heads

How Magnetic Fields Effect Hard Drives

Abstract

While it is obvious that hard drives can tolerate the Earth’s relatively weak magnetic field, their ability to reliably store and transfer data in the presence of stronger magnetic fields is less well known. Two different models of hard drives were tested in non-time varying magnetic fields of various strengths. The impact of the magnetic flux exposure was quantified by performing data transfer rate benchmark testing, data integrity tests, predictive testing, and surface scanning on the drives before, during and after exposure.  The drives were found to be largely immune to magnetic flux densities up to approximately 250 Gauss (0.025 Tesla).

Introduction

The first magnetic hard disk drive was created by IBM in 1956. Despite extreme improvements in data density and data transfer rates, the fundamental concept of operation in modern drives remains largely intact (Hayes, 2002).  The drives store and retrieve data using movable read and write heads held proximal to rotating platters. The platters are coated with a thin layer of cobalt alloy (previously an iron-based magnetic material) which is divided into magnetic domains called “bit cells”. A bit cell in a modern drive contains 50 to 100 grains of magnetic material, and the collective magnetic orientation of these grains in a single bit cell represents a logical binary “0” or “1”. Through the read/write heads, the disk drive has the means to both detect previously written ones and zeroes (read), and to create or reverse magnetic polarization in bit cells to create new stored data (write).

Hard Drive Heads - Susceptibility of Magnetic Fields

The actual mechanisms for reading and writing are entirely different. The writing operation is performed by an electromagnet which has a core designed to concentrate intense magnetic flux on individual bit cells as the head “flies” over the cells in the spinning platter.  The current applied to the electromagnet in the write head can be applied in either polarity, thereby establishing a magnetic south or north pole at the writing point, and consequently storing either a one or a zero. On older drives the read operation was performed by an inductive pickup coil, essentially performing the write operation in reverse. When a coil (read head) passes through a magnetic field, a current is induced according to Faraday’s law of induction. In the 1990s IBM invented the magnetoresistive head, which removed a barrier to further data density increases in the years that would follow. This method uses materials in the read head that change their resistance according to magnetic flux exposure. Therefore, the contents of successive bit cells can be read by monitoring the pattern of resistance changes provided by the read head.  Both the read and write operations benefit from technology that allows the heads to fly on a layer of air only 10-20 nm above the surface of the platter (Hayes, 2002).

 Understanding Vulnerabilities

The inner workings of hard disk drives must be understood to consider vulnerabilities for data loss due to externally applied magnetic fields.  Knowing that an electromagnet stores data in the bit cells by passing magnetic flux through them suggests that some externally applied magnetic field strength of opposing direction might be able to cancel the intended field from the write head, thereby interfering with the write operation.  After the data is stored, it is reasonable to expect that a strong enough interfering magnetic field could reverse the orientation of some or all of the grains in a bit cell, possibly converting a zero to a one or vice versa, and corrupting the data.  The ability of the magnetic material to resist this phenomenon is known as coercivity, which has been well understood since the late 19th century.  Pure iron has a very low coercivity of ~2 Oe (160 A/m), (Thompson, 1896), whereas  modern magnetic media with Cobalt based coatings in disk drives can be on the order of 1000 times higher (Yang, 1991).

 Magnetic Fields in Data Centers

In the commercial data center industry, it is common knowledge that the value of stored data itself far exceeds that of the hardware upon which it is stored.  Today’s hard drives are designed to last about 5 years, measured by the Component Design Life (CDL) method.  Clearly a business process may rely on a database to generate revenue, but where the database resides is of little importance.  Consequently, enterprise data center owners go to great lengths to protect their data, sometimes constructing entire mirrored, geographically diverse facilities to reduce the probability of data loss.  It follows then that users of magnetic disk drives in such environments are understandably nervous about risks of data loss and corruption.

One such perceived risk is that associated with magnets being stored or used near magnetic storage media. Historically, this risk may have been well founded. The earliest hard disks such as IBM’s 1956 model, used iron oxide as the magnetic material. The low coercivity of iron compounds means there would have been little resistance to data corruption from external magnetic fields. Iron compounds were still commonly used in the ubiquitous floppy disks of the 1980’s and 1990’s. Floppy disks are known for being vulnerable to damage from even relatively weak magnets (Keizer, 2004). This may have contributed to a persisting fear of magnets in the vicinity of modern hard drives. Small rare earth magnets are commonly used in consumer products, and their usage is increasing. Apple Inc.’s iPad 2 product contains 31 magnets, 21 in the device’s esteemed folding cover, and another 10 in the iPad itself.  As of the date of this paper, Apple has sold over 60 million iPads. In 2011, 92% of fortune 500 companies were testing or deploying iPads (Wingfield, 2011), yet the servers, laptops and hard drives in proximity to all these iPad magnets are not failing.

Magnets are also showing up in data centers. Companies have developed products such as thermal containment systems that are designed to be mounted directly to IT equipment enclosures with magnets. This is thought to be advantageous, because other methods such as mechanical fastening and adhesives involve more labor and are problematic due to metal shavings and adhesive aging/failure. One objection to this recent industry trend has been concern over risk to magnetic storage media. This study determines whether magnets really pose a risk to modern hard drives, and if so, at what field strength.

The complete article was recently published in IEEE Potentials magazine. It may be downloaded here.

Still not convinced? Maybe you saw the Breaking Bad Episode where the protagonists destroyed evidence on hard drives by parking a giant mobile electromagnet outside the locked police evidence room?  Well, lets just say to get above 250 gauss from 10 feet away, they would have needed more than a few car batteries strung together in the back of a truck, but an amusing episode nonetheless. Hollywood – if you are reading, Kleinholz Inc. is available for technical consults, should any producers wish to form a stronger allegiance with scientific reality. Interestingly, EMI attacks on data centers are a real, and often unguarded threat today but that’s a topic for another blog post.