The upper and lower cases house 10 0.5mm-thick substrates and 12 0.381mm-thick substrates, respectively. (source: Hoya)
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Continued from Hoya's Glass Substrates Hold Key to 20TB HDDs (1)

An existing 12-Tbyte product of WD stores eight disks (thickness: 0.635mm each). Even with the same recording density, 10 and 12 disks will realize 15- and 18-Tbyte capacities, respectively *1.

*1: When 12 disks and so-called SMR (shingled magnetic recording) method are used, it becomes possible to increase the capacity to 20 Tbytes. The SMR is a technology that narrows effective track width by partially superimposing data-storing tracks on adjacent tracks. Because there is a limit to data writing, it is offered as a different product line at this point.

As the number of disks is increased to increase capacity, parts costs increase as well. Still, this is one of the effective methods to increase HDD capacity because the increase in recording density is slower than before.

Even when the costs increase to some degree, a large demand is expected at data centers, which are major customers. It is because, when recording capacity is increased without changing device size, it becomes possible to cope with an increase in the amount of data without making changes to facilities such as racks.

For Hoya, when the number of disks per HDD increases, the demand for disk substrates grows.

Standard technology for heat assisted magnetic recording

Another advantage of glass substrates is that they are crucial to heat assisted magnetic recording (HAMR), which is expected to be the mainstream in the future. The HAMR is a technology to record data in a minute area heated by a laser light and requires a substrate with a heat resistance of about 700°C.

The heat resistance of existing aluminum substrates is said to be about 200°C. So, it is difficult to deal with the problem without using glass substrates.

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