All of the early Cray mainframes (even the Cray-1 discussed here, although maybe only the somewhat later Cray-1/S) supported a "Solid State Disk" peripheral, which was basically a refrigerator full of RAM that was accessible via DMA channel. The intent was to be used to swap in and out large data sets from memory - I think even the early ones went up to a gigabyte in size.
Yes, the Cray Solid State Storage Device (SSD) was a standalone unit that looked like a quarter of a Cray, complete with bench. It held up to 2 gigabytes of data in MOS memory and was accessed via an I/O channel at 100 megabytes to 1.25 gigabytes per second. It was used for "temporary storage of datasets" and could "significantly increase data transfer rates." The SSD was separate from the Cray's main memory.
