Non-removable-media disk

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Non-removable-media disks were the generation of magnetic disk mass storage peripherals after the previously ubiquitous removable-pack disk‎s. They used 'head/disk assemblies' (HDAs), which packaged the platters and the heads in a pre-assembled, sealed (in 'clean-room' conditions), unit. The sealed HDA was, and remains, a key step in improving the storage density (and thus capacity) of disk drives.

The technical rationale for using sealed HDAs has several parts, but an important one is as follows. To maximize storage density, the head must be 'flown' as close to the surface of the platter as possible; at such heights, even tiny dust particles can cause a head crash. If outside air is being introduced, it is difficult to filter out the smallest particles; and in any event, the filters must be replaced regularly, a maintenance headache. Sealing the HDA in a 'clean room' environment avoids these problems.

It was later discovered that filling the HDA with helium was advantageous, due to its lower density; that reduces turbulence around the head.

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