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MB (Megabyte)
One megabyte as 1,000,000 (one million) bytes.

MFM (Multiple Frequency Modulation)
A method of encoding analog signals into magnetic pulses or bits.

MR Heads (Magneto-resistive Heads)
MR heads were developed to increase area density and improve drive performance. MR heads use separate read and write elements, as opposed to traditional inductive thin-film read-write heads. MR heads use an inductive element for writing data, and a separate magneto-resistive element for reading information. The read element has a magnetically sensitive material that detects data recorded on the magnetic disk surface. MR head construction results in a stronger signal than that produced by inductive thin-film read-write heads, which permits it to read higher area density data. Since the magneto-resistive element can only read data, a conventional thin-film inductive element writes data to the disk.

MTBF (Mean Time Between Failure)
A measure of reliability. The MTBF is the number of failures divided by the number of hours the component has operated. The longer the time span between failures, the more reliable the device.

MTTR (Mean Time to Repair)
The average time it takes to repair a drive in the field. In the field, only major subassemblies are changed (the PCB, sealed housing, etc.), excluding component level repairs as these are not performed in the field.

Magnetic Flux
The pattern of magnetic pole directions of the bits written on the disk.

Manufacturing Yield
The portion of unit production of a manufacturing process that is usable, saleable product; usually expressed as a percent of total output of that product.

Master
The first drive in a dual drive combination. A master drive by itself (with no slave) is called a single drive.

Media
In hard drives, the disks and their magnetic coatings; sometimes refers to the coating material alone.

Mechanical Latencies
Include both seek time and rotational latency. Mechanical latencies are the main hindrance to higher performance in hard drives. The time delays of mechanical latencies are one hundred times higher than electronic (non-mechanical) latencies associated with the transferring of data. See also Seek Time, Rotational Latency.

Memory
A device or storage system capable of storing and retrieving data.

Millisecond (ms)
1/1,000 (one-thousandth) of a second. Hard disks are rated in milliseconds. Higher numbers mean slower performance.

Multi-media
A simultaneous presentation of data in more than one form, such as by means of both visual and audio.

Multi-user
In information technology, a system that enables more than one user to access data at the same time.