History (1978): Probably First SSD

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1978: STC 4305
First solid state disk drive

Why it’s important
The last instance of mainframe subsystem performance improvement by dedicated ‘gap filling’ high performance hardware. In this case, the 4305 targeted the fixed head disk drives (i.e., 2305). Subsequent caching in the subsystem eliminated the need for such performance enhancement products.

Discussion
It is not clear that this is the first solid state disk drive.

At least Alpha Data may have offered such a product in 1977.

It is possible that IBM Defense Systems used IBM Bulk Store as a DASD.

Other early defense products may have used core as a disk.