Hi DanO1
I think you are getting confused with the term Disc Streaming, which was introduced with the Gigasampler Software around the late 90s. (Cant remember the exact year)
Here is a general description.
Most sounds in keyboards etc use samples, which are recordings of different parts of real instruments, and so when you press the key you actually play back a recording of the instrument, the more samples that can be taken the more accurate the sound becomes, (IE instead of sampling each octave of a piano you sample each key) unfortunately this also requires more memory, (Which with high quality Piano samples can be 2GB or more just for 1 voice) now as an arranger has 100s of voices, this would require masses amount of memory (And is currently not practical) to get really top notch sounds to play in real time. (Other methods are now starting to become available)
A hard disk can have masses amount of storage capacity, but it takes time to get the sound off the disk, which means real time play is not possible.
What Giga sampler introduced was a technique that loaded the first part of the sound of each note permanently into Ram, giving the Hard Disk time to load (Stream) in the rest of the sample, which then allows the use of multiple high quality samples in real time.
All this was done on standard computer systems, and so could not be loaded into bespoke hardware arrangers, but could be incorporated into software arrangers, (Which contain a standard computer) the only physical arrangers that I know of at present that can do this, are Wersi and Lionstracs. (Disc streaming on the Wersi became an option in about 2001/2002)
Hope this helps

Bill