1990	IBM Sawmill
	1st MR head drive & 1st negative air pressure bearing

Why it’s important
The minimum transition width between flux reversals on a disk is inversely proportional to the disk's coercivity. Increasing disk coercivity, up to the limit of the write field achievable form the recording head allows for increasing the packing density of information along the track of a disk. One of the most important advances in HDD technology was commercialization of dual element film recording heads. Such heads use one element optimized for writing data onto a disk and a second element optimized for reading data from the disk. The ability to separately optimize these head functions allowed achieving much higher head write fields, capable of writing data onto media of much higher coercivity than would have been possible with extensions of single element ferrite or film heads.

The IBM Sawmill marks the beginning of the use of magneto-resistive read element in heads replacing the inductive heads used heretofore. MR heads respond directly to the recorded bits allowing the bits to be closer together and thereby increasing capacity.

The minimum transition width between flux reversals on a disk is inversely proportional to the disk's coercivity. Increasing disk coercivity, up to the limit of the write field achievable form the recording head allows for increasing the packing density of information along the track of a disk.

Negative air pressure heads literally are both repelled and sucked towards the spinning disk resulting in lower stable heights than the previous flying heads.

During the 1990’s MR heads, the their improved GMR (for Giant) heads both with negative air pressure bearings displaced the prior thin film and ferrite (inductive) flying heads.

MR technology was first proposed for digital magnetic recording in 1971 by J.R, Hunt, see IEEE TransMag MAG-7 the effect was discovered by Lord Kelvin circa 1857 [source: Design and Analysis of Magnetoresisive Recording Heads, Williams, Wiley & Son, 2001, p.20]

As the 9345 DASD the Sawmill disk drive was initially was available as a PRPQ as part of SCSE (SuperComputing Systems Extensions) beginning November 1990. It then shipped with the IBM Model 9570 high-speed array device and subsequent as the DASD for the Models 9341 and 9343 low-end mainframe systems.

It was a 5¼” disk drive, the 9345 DASD Model 1 had two 1.0 GB HDAs while the Model 2 had two 1.5 GB HDAs.

Discussion

Al Shugart in a 2000 interview identified MR heads as one of the four most significant events in the history of mass storage.

Additional information

Head actuatordynamics of an IBM 5¼-inch disk drive, DP Fazio et al., IBM JRD, July 1993
IBM 9345 DASD in CHM collection.
IBM 9345 DASD, partially cut away in CHM collection
Exploded IBM Sawmill drive, display case and a specifications sheet, in CHM collection.