IBM Sawmill

1990 IBM Sawmill
First magnetoresistive (MR) head drive and first negative air pressure bearing

Why it’s important

The IBM Sawmill marks the beginning of the use of magneto-resistive read element in heads with negative air pressure air bearings. MR heads respond directly to the recorded bits allowing the bits to be closer together. Negative air pressure heads literally are both repelled and sucked towards the spinning disk resulting in lower stable heights which in turn enables higher bit density. The two technologies thereby increasing capacity.

During the 1990’s MR heads with negative air pressure bearings displaced the prior thin film and ferrite (inductive) flying heads with positive air pressure bearings.


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

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.

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.

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